Document:

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                                                               EXECUTION VERSION

                                                                     EXHIBIT 4.2

                          REGISTRATION RIGHTS AGREEMENT

         REGISTRATION RIGHTS AGREEMENT (this "AGREEMENT"), dated as of January
30, 2004, by and between Zix Corporation, a Texas corporation, with its
headquarters located at 2711 North Haskell Avenue, Suite 2300 LB36, Dallas,
Texas 75204 (the "COMPANY"), Aventis Inc., a Pennsylvania corporation
("AVENTIS") and Aventis Holdings Inc., a Delaware corporation ("AVENTIS
HOLDINGS" and, together with Aventis, the "SELLING SHAREHOLDERS").

         WHEREAS:

         A.       In connection with an Asset Purchase Agreement among the
Company, MyDocOnline, Inc. ("MYDOC"), Aventis Pharmaceuticals Holdings Inc. and
Aventis Pharmaceuticals Inc. dated of even date herewith (the "ASSET PURCHASE
AGREEMENT"), the Company has agreed, upon the terms and subject to the
conditions contained therein, to issue and sell to MyDoc or its designee 583,411
shares of its Common Stock (the "SHARES"), par value $.01 per share (the "COMMON
STOCK"), and to issue to MyDoc or its designee a Warrant (the "WARRANT")
exercisable for 145,853 additional shares of Common Stock, in each case as
referenced in Section 4.1 of the Asset Purchase Agreement; and

         B.       MyDoc has directed the Company to issue the Shares and the
Warrant to Aventis Holdings; and

         C.       In connection with the Asset Purchase Agreement, Aventis has
agreed to loan the Company $3,000,000 (the "LOAN") as evidenced by that certain
Promissory Note dated of even date herewith (the "NOTE") and the Company may
issue shares of its Common Stock to Aventis as repayment of the Note; and

         D.       To induce MyDoc to execute and deliver the Asset Purchase
Agreement and to induce Aventis to make the Loan to the Company, the Company has
agreed to provide certain registration rights under the Securities Act of 1933,
as amended, and the rules and regulations thereunder, or any similar successor
statute (collectively, the "1933 ACT"), and applicable state securities laws;

         NOW, THEREFORE, in consideration of the premises and the mutual
covenants contained herein and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, the Company and each
of the Selling Shareholders hereby agree as follows:

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                                   ARTICLE I.
                                   DEFINITIONS

         Section 1.1. As used in this Agreement, the following terms shall have
the following meanings:

                  (a)      "REGISTER," "REGISTERED," and "REGISTRATION" refer to
         a registration effected by preparing and filing a Registration
         Statement or Statements in compliance with the 1933 Act and pursuant to
         Rule 415 under the 1933 Act or any successor rule providing for
         offering securities on a continuous basis ("RULE 415"), and the
         declaration or ordering of effectiveness of such Registration Statement
         by the United States Securities and Exchange Commission (the "SEC").

                  (b)      "REGISTRABLE SECURITIES" means (i) the Shares being
         issued to Aventis Holdings pursuant to the Asset Purchase Agreement,
         (ii) the shares of Common Stock issuable to Aventis Holdings upon
         exercise of the Warrant, (iii) the shares of Common Stock issuable to
         Aventis upon repayment of the Note and (iv) any Common Stock issued as
         a dividend or other distribution with respect to, or in exchange for or
         in replacement of, such above-described securities.

                  (c)      "REGISTRATION STATEMENT(S)" means a registration
         statement(s) of the Company under the 1933 Act.

         Section 1.2. Capitalized terms used herein and not otherwise defined
herein shall have the respective meanings set forth in the Asset Purchase
Agreement.

                                  ARTICLE II.
                                  REGISTRATION

         Section 2.1. REGISTRATION. The Company shall prepare, and file with the
SEC within thirty (30) days of the Closing Date (as defined in the Asset
Purchase Agreement) (the "FILING DATE") a Registration Statement on Form S-3
(or, if Form S-3 is not then available, on such form of Registration Statement
as is then available) to effect a registration of the applicable Registrable
Securities, subject to the consent of Aventis Holdings (which consent will not
be unreasonably withheld, conditioned or delayed) covering the resale of such
Registrable Securities. The Company shall provide a draft of the Registration
Statement to counsel for the applicable Selling Shareholder prior to its filing
or other submission so that such counsel has a reasonable opportunity to review
and comment on such Registration Statement prior to it being filed. The Company
shall use all commercially reasonable efforts to obtain effectiveness of the
Registration Statement as soon as possible thereafter.

         Section 2.2. UNDERWRITTEN OFFERING. If any offering pursuant to a
Registration Statement pursuant to Section 2.1 hereof involves an underwritten
offering, the Selling Shareholders shall have the right to select one legal
counsel and the right to select the investment banker or bankers and manager or
managers to administer the offering (subject to the approval of the Company,
which approval shall not be unreasonably withheld, conditioned or delayed). In
the event that a Selling Shareholder elects not to participate in such
underwritten offering, the

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Registration  Statement covering all of the Registrable Securities shall contain
appropriate  plans  of  distribution  reasonably  satisfactory  to such  Selling
Shareholder.

         Section 2.3. ELIGIBILITY FOR FORM S-3. The Company represents and
warrants that, to the best of its knowledge, it meets the registrant eligibility
and transaction requirements for the use of Form S-3 for registration of the
sale by the Selling Shareholders and the Company shall file all reports required
to be filed by the Company with the SEC in a timely manner so as to maintain
such eligibility for the use of Form S-3.

                                  ARTICLE III.
                           OBLIGATIONS OF THE COMPANY

         In connection with the registration of the Registrable Securities, the
Company shall have the following obligations:

         Section 3.1. The Company shall prepare, file with the SEC and obtain
effectiveness for a Registration Statement as provided in Section 2.1, and
thereafter use its commercially reasonable efforts to keep the Registration
Statement effective pursuant to Rule 415 at all times until such date as is the
earlier of (i) the date on which all of the Registrable Securities have been
sold and (ii) the date on which the Registrable Securities (in the opinion of
Company's counsel) may be immediately sold to the public without registration or
restriction (including without limitation as to volume by each holder thereof)
under the 1933 Act (the "REGISTRATION PERIOD"), which Registration Statement
(including any amendments or supplements thereto and prospectuses contained
therein) shall not contain any untrue statement of a material fact or omit to
state a material fact required to be stated therein, or necessary to make the
statements therein not misleading.

         Section 3.2. The Company shall prepare and file with the SEC such
amendments (including post-effective amendments) and supplements to the
Registration Statements and the prospectus used in connection with the
Registration Statements as may be necessary to keep the Registration Statements
effective at all times during the Registration Period, and, during such period,
comply with the provisions of the 1933 Act with respect to the disposition of
all Registrable Securities of the Company covered by the Registration
Statements, including any amendments that may be required to allow the permitted
assignees of the Selling Shareholders (as set forth in Section 10.7) to sell
Registrable Securities under the Registration Statements.

         Section 3.3. The Company shall furnish to each Selling Shareholder
whose Registrable Securities are included in a Registration Statement and its
legal counsel (i) promptly after the same is prepared and publicly distributed,
filed with the SEC, or received by the Company, one copy of each Registration
Statement and any amendment thereto, each preliminary prospectus and prospectus
and each amendment or supplement thereto, and (ii) such number of copies of a
prospectus, including a preliminary prospectus, and all amendments and
supplements thereto and such other documents as such Selling Shareholder may
reasonably request in order to facilitate the disposition of the Registrable
Securities owned by such Selling Shareholder. The Company will immediately
notify such Selling Shareholder of the effectiveness of each Registration
Statement or any post-effective amendment. The Company will promptly respond to
any and all comments received from the SEC, with a view towards causing each
Registration Statement or

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any amendment thereto to be declared effective by the SEC as soon as practicable
and shall file an acceleration request as soon as practicable following the
resolution or clearance of all SEC comments or, if applicable, following
notification by the SEC that any such Registration Statement or any amendment
thereto will not be subject to review.

         Section 3.4. The Company shall use reasonable efforts to (i) register
and qualify the Registrable Securities covered by the Registration Statements
under such other securities or "BLUE SKY" laws of such jurisdictions in the
United States as each Selling Shareholder may reasonably request, if an
exemption from such securities or blue sky laws is not otherwise available, (ii)
prepare and file in those jurisdictions such amendments (including
post-effective amendments) and supplements to such registrations and
qualifications as may be necessary to maintain the effectiveness thereof during
the Registration Period, (iii) take such other actions as may be necessary to
maintain such registrations and qualifications in effect at all times during the
Registration Period, and (iv) take all other actions reasonably necessary or
advisable to qualify the Registrable Securities for sale in such jurisdictions;
provided, however, that the Company shall not be required in connection
therewith or as a condition thereto to (a) qualify to do business in any
jurisdiction where it would not otherwise be required to qualify but for this
Section 3.4, (b) subject itself to general taxation in any such jurisdiction,
(c) file a general consent to service of process in any such jurisdiction, or
(d) make any change in its charter or bylaws, which in each case the Board of
Directors of the Company determines to be contrary to the best interests of the
Company and its shareholders.

         Section 3.5. In the event a Selling Shareholder selects underwriters
for the offering, the Company shall enter into and perform its obligations under
an underwriting agreement, in usual and customary form, including, without
limitation, customary indemnification and contribution obligations, with the
underwriters of such offering.

         Section 3.6. As promptly as practicable after becoming aware of such
event, the Company shall notify each Selling Shareholder of the happening of any
event, of which the Company has knowledge, as a result of which the prospectus
included in any Registration Statement, as then in effect, includes an untrue
statement of a material fact or omission to state a material fact required to be
stated therein or necessary to make the statements therein not misleading, and
use its commercially reasonable efforts promptly to prepare a supplement or
amendment to any Registration Statement to correct such untrue statement or
omission, and deliver such number of copies of such supplement or amendment to
such Selling Shareholder as such Selling Shareholder may reasonably request;
provided, however, that, for not more than fifteen (15) consecutive trading days
(or a total of not more than thirty (30) trading days in any twelve (12) month
period), the Company may delay the disclosure of material non-public information
concerning the Company (as well as prospectus or Registration Statement
updating) the disclosure of which at the time is not, in the good faith opinion
of the Company, in the best interests of the Company (an "ALLOWED DELAY");
provided, further, that the Company shall promptly (i) notify such Selling
Shareholder in writing of the existence of (but in no event, without the prior
written consent of such Selling Shareholder, shall the Company disclose to such
Selling Shareholder any of the facts or circumstances regarding) material
non-public information giving rise to an Allowed Delay and (ii) advise such
Selling Shareholder in writing to cease all sales under such Registration
Statement until the end of the Allowed Delay. Upon

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expiration of the Allowed Delay, the Company shall again be bound by the first
sentence of this Section 3.6 with respect to the information giving rise
thereto.

         Section 3.7. The Company shall use its commercially reasonable efforts
to prevent the issuance of any stop order or other suspension of effectiveness
of any Registration Statement, and, if such an order is issued, to obtain the
withdrawal of such order at the earliest possible moment and to notify each
Selling Shareholder who holds Registrable Securities being sold (or, in the
event of an underwritten offering, the managing underwriters) of the issuance of
such order and the resolution thereof.

         Section 3.8. The Company shall permit a single firm of counsel
designated by the applicable Selling Shareholder to review such Registration
Statement, and all amendments and supplements thereto (as well as all requests
for acceleration or effectiveness thereof (collectively, the "REGISTRATION
DOCUMENTS")) a reasonable period of time prior to their filing with the SEC, and
not file (or send) any Registration Documents in a form to which such counsel
reasonably objects and will not request acceleration of such Registration
Statement without prior notice to such counsel. The sections of such
Registration Statement covering information with respect to such Selling
Shareholder, such Selling Shareholder's beneficial ownership of securities of
the Company or such Selling Shareholder's intended method of disposition of
Registrable Securities shall conform to the information provided to the Company
by such Selling Shareholder.

         Section 3.9. The Company shall (i) cause all the Registrable Securities
covered by the Registration Statement to be listed on each national securities
exchange on which securities of the same class or series issued by the Company
are then listed, if any, if the listing of such Registrable Securities is then
permitted under the rules of such exchange, or (ii) to the extent the securities
of the same class or series are not then listed or permitted to be listed on a
national securities exchange, secure the designation and quotation of all the
Registrable Securities covered by the Registration Statement on the Nasdaq
automated quotation system.

         Section 3.10. The Company shall provide a transfer agent and registrar,
which may be a single entity, for the Registrable Securities not later than the
effective date of the Registration Statement.

         Section 3.11. At the request of the applicable Selling Shareholder, the
Company shall prepare and file with the SEC such amendments (including
post-effective amendments) and supplements to a Registration Statement and any
prospectus used in connection with the Registration Statement as may be
necessary in order to change the plan of distribution set forth in such
Registration Statement.

         Section 3.12. The Company shall not, and shall not agree to, allow the
holders of any securities of the Company to include any of their securities in
any Registration Statement under Section 2.1 hereof or any amendment or
supplement thereto under Section 3.2 hereof without the consent of the
applicable Selling Shareholder. In addition, the Company shall not offer any
securities for its own account or the account of others in any Registration
Statement under Section 2.1 hereof or any amendment or supplement thereto under
Section 3.2 hereof without the consent of such Selling Shareholder.

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         Section 3.13. The Company shall take all other reasonable actions
necessary to expedite and facilitate disposition by the applicable Selling
Shareholder of Registrable Securities pursuant to a Registration Statement.

         Section 3.14. The Company shall comply with all applicable laws related
to a Registration Statement and offering and sale of securities and all
applicable rules and regulations of governmental authorities in connection
therewith (including without limitation the 1933 Act and the 1934 Act (as
defined below) and the rules and regulations promulgated by the SEC).

         Section 3.15. If (i) a Registration Statement is not filed by the
Filing Date, (ii) the Registration Statement is not declared effective by the
SEC by the close of business on the 120th day following the Closing Date or
(iii) once declared effective by the SEC, sales of the Registrable Securities
cannot be made (other than during an Allowed Delay, as defined above) pursuant
to the Registration Statement through no fault of the applicable Selling
Shareholder, unless the Company is notified by a Selling Shareholder of its
election not to receive the liquidated damages provided by this Section 3.15 (in
accordance with Section 10.5), then the Company shall pay to each applicable
Selling Shareholder, as liquidated damages and not as a penalty, an amount in
cash equal to (A) US $2,225 times (B) (x) the total number of Registrable
Securities held by such holder (which number shall exclude any Registrable
Securities issued pursuant to the Note) divided by (y) the total number of
Registrable Securities held by the Selling Shareholders (which number shall
exclude any Registrable Securities issued pursuant to the Note), for each day
the Registration Statement is not filed, effective or available for sale, as
applicable, payable on the last business day of each month. In addition to the
foregoing, if the Registration Statement is not declared effective by the SEC by
the close of business on the 180th day following the Closing Date, the Minimum
Annual Requirements (as defined in the Master Services Agreement between the
Company and Aventis dated of even date herewith, the "SERVICES AGREEMENT") shall
automatically be reduced by 50% without further action by either party. For
purposes of illustration, if the conditions in the preceding sentence are met,
an obligation of $1,000,000 shall be reduced to $500,000. The foregoing payment
and reduction of the Minimum Annual Requirements shall be exclusive of any other
remedy for money damages available to such Selling Shareholder for failure of
the Company to comply with its obligations under this Agreement.

                                  ARTICLE IV.
                     OBLIGATIONS OF THE SELLING SHAREHOLDER

         In connection with the registration of the Registrable Securities, each
Selling Shareholder shall have the following obligations:

         Section 4.1. It shall be a condition precedent to the obligations of
the Company to complete the registration pursuant to this Agreement with respect
to the Registrable Securities of each Selling Shareholder that such Selling
Shareholder shall furnish to the Company such information regarding itself, the
Registrable Securities held by it and the intended method of disposition of the
Registrable Securities held by it as shall be reasonably required to effect the
registration of such Registrable Securities and shall execute such documents in
connection with such registration as the Company may reasonably request. At
least ten (10) days prior to the first

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anticipated filing date of the Registration Statement, the Company shall notify
the applicable Selling Shareholder of the information the Company requires from
such Selling Shareholder.

         Section 4.2. Each Selling Shareholder, by such Selling Shareholder's
acceptance of the Registrable Securities, agrees to cooperate with the Company
as reasonably requested by the Company in connection with the preparation and
filing of the Registration Statements hereunder, unless such Selling Shareholder
has notified the Company in writing of such Selling Shareholder's election to
exclude all of such Selling Shareholder's Registrable Securities from the
Registration Statements.

         Section 4.3. If the services of an underwriter are engaged, the
applicable Selling Shareholder agrees to enter into and perform such Selling
Shareholder's obligations under an underwriting agreement, in usual and
customary form, including, without limitation, customary indemnification and
contribution obligations, with the managing underwriter of such offering and
take such other actions as are reasonably required in order to expedite or
facilitate the disposition of the Registrable Securities, unless such Selling
Shareholder has notified the Company in writing of such Selling Shareholder's
election to exclude all of such Selling Shareholder's Registrable Securities
from such Registration Statement.

         Section 4.4. Each Selling Shareholder agrees that, upon receipt of any
notice from the Company of the happening of any event of the kind described in
Section 3.6 or 3.7, such Selling Shareholder will immediately discontinue
disposition of Registrable Securities pursuant to the Registration Statement
covering such Registrable Securities until such Selling Shareholder's receipt of
the copies of the supplemented or amended prospectus contemplated by Section 3.6
or 3.7 or until it is advised in writing (an "ADVICE") by the Company that the
use of the applicable prospectus may be resumed, and, if so directed by the
Company to discontinue disposition, such Selling Shareholder shall deliver to
the Company or destroy all copies in such Selling Shareholder's possession, of
the prospectus covering such Registrable Securities current at the time of
receipt of such notice.

         Section 4.5. No Selling Shareholder may participate in any underwritten
registration hereunder unless such Selling Shareholder (i) agrees to sell such
Selling Shareholder's Registrable Securities on the basis provided in any
underwriting arrangements in usual and customary form entered into by the
Company, (ii) completes and executes all questionnaires, powers of attorney,
indemnities, underwriting agreements and other documents reasonably required
under the terms of such underwriting arrangements, and (iii) agrees to pay its
pro rata share of all underwriting discounts and commissions and any expenses in
excess of those payable by the Company pursuant to Article 5 below.

         Section 4.6. Each Selling Shareholder shall comply with all applicable
laws related to a Registration Statement and offering and sale of securities and
all applicable rules and regulations of governmental authorities in connection
therewith (including without limitation the 1933 Act and the 1934 Act (as
defined below) and the rules and regulations promulgated by the SEC).

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                                   ARTICLE V.
                            EXPENSES OF REGISTRATION

         All reasonable expenses, other than underwriting discounts and
commissions, incurred in connection with registrations, filings or
qualifications pursuant to Articles 2 and 3, including, without limitation, all
registration, listing and qualification fees, printers and accounting fees, the
fees and disbursements of legal counsel for the Company, and the reasonable fees
and disbursements of one legal counsel for the Selling Shareholders, selected by
the Selling Shareholders pursuant to Sections 2.2 and 3.8 hereof, shall be borne
by the Company.

                                  ARTICLE VI.
                                 INDEMNIFICATION

         In the event any Registrable Securities are included in a Registration
Statement under this Agreement:

         Section 6.1. To the extent permitted by law, the Company will
indemnify, hold harmless and defend (i) each Selling Shareholder who holds such
Registrable Securities, (ii) the directors, officers, partners, employees,
agents and each person who controls such Selling Shareholder within the meaning
of the 1933 Act or the Securities Exchange Act of 1934, as amended (the "1934
ACT"), if any, (iii) any underwriter (as defined in the 1933 Act) for such
Selling Shareholder, and (iv) the directors, officers, partners, employees and
each person who controls any such underwriter within the meaning of the 1933 Act
or the 1934 Act, if any (each, an "INDEMNIFIED PERSON"), against any joint or
several losses, claims, damages, liabilities or expenses (collectively, together
with actions, proceedings or inquiries by any regulatory or self-regulatory
organization, whether commenced or threatened, in respect thereof, "CLAIMS") to
which any of them may become subject insofar as such Claims arise out of or are
based upon: (i) any untrue statement or alleged untrue statement of a material
fact in a Registration Statement or the omission or alleged omission to state
therein a material fact required to be stated or necessary to make the
statements therein not misleading; (ii) any untrue statement or alleged untrue
statement of a material fact contained in any preliminary prospectus if used
prior to the effective date of such Registration Statement, or contained in the
final prospectus (as amended or supplemented, if the Company files any amendment
thereof or supplement thereto with the SEC) or the omission or alleged omission
to state therein any material fact necessary to make the statements made
therein, in light of the circumstances under which the statements therein were
made, not misleading; or (iii) any violation or alleged violation by the Company
of the 1933 Act, the 1934 Act, any other law, including, without limitation, any
state securities law, or any rule or regulation thereunder relating to the offer
or sale of the Registrable Securities (the matters in the foregoing clauses (i)
through (iii) being, collectively, "VIOLATIONS"). Subject to the restrictions
set forth in Section 6.3 with respect to the number of legal counsel, the
Company shall reimburse the Indemnified Person, promptly as such expenses are
incurred and are due and payable, for any reasonable legal fees or other
reasonable expenses incurred by them in connection with investigating or
defending any such Claim. Notwithstanding anything to the contrary contained
herein, the indemnification agreement contained in this Section 6.1: (i) shall
not apply to a Claim by an Indemnified Person arising out of or based upon a
Violation which occurs in reliance upon and in conformity with information
furnished in writing to the Company by such Indemnified Person or underwriter
for such Indemnified Person expressly for use in connection with the preparation
of such Registration Statement or any such amendment thereof or supplement
thereto; (ii) shall not apply to amounts paid in settlement of any Claim if such
settlement is

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effected without the prior written consent of the Company, which consent shall
not be unreasonably withheld; (iii) with respect to any preliminary prospectus,
shall not inure to the benefit of any Indemnified Person if the untrue statement
or omission of material fact contained in the preliminary prospectus was
corrected on a timely basis in the prospectus, as then amended or supplemented,
such corrected prospectus was timely made available by the Company pursuant to
Section 3.3 hereof, and the Indemnified Person was promptly advised in writing
not to use the incorrect prospectus prior to the use giving rise to a Violation
and such Indemnified Person, notwithstanding such advice, used it; and (iv)
shall not apply to any offers to sell or sales made by an Indemnified Person
after receipt of the notice referred to in Section 3.6, clause (ii) and during
the period of the Allowed Delay. Such indemnity shall remain in full force and
effect regardless of any investigation made by or on behalf of the Indemnified
Person and shall survive the transfer of the Registrable Securities by the
Selling Shareholder pursuant to Section 10.7.

         Section 6.2. In connection with any Registration Statement in which a
Selling Shareholder is participating, such Selling Shareholder agrees to
indemnify, hold harmless and defend, to the same extent and in the same manner
set forth in Section 6.1, the Company, each of its directors, each of its
officers who signs the Registration Statement, each person, if any, who controls
the Company within the meaning of the 1933 Act or the 1934 Act, any underwriter
and any other shareholder selling securities pursuant to the Registration
Statement or any of its directors or officers or any person who controls such
shareholder or underwriter within the meaning of the 1933 Act or the 1934 Act
(collectively and together with an Indemnified Person, an "INDEMNIFIED PARTY"),
against any Claim to which any of them may become subject, under the 1933 Act,
the 1934 Act or otherwise, insofar as such Claim arises out of or is based upon
(i) any Violation, in each case to the extent (and only to the extent) that such
Violation occurs in reliance upon and in conformity with written information
furnished to the Company by such Selling Shareholder expressly for use in
connection with such Registration Statement or (ii) any offers to sell or sales
by such Selling Shareholder after receipt by such Selling Shareholder of the
notice referred to in Section 3.6, clause (ii) and during the period of the
Allowed Delay; and subject to Section 6.3, such Selling Shareholder will
reimburse any legal or other expenses (promptly as such expenses are incurred
and are due and payable) reasonably incurred by them in connection with
investigating or defending any such Claim; provided, however, that the indemnity
agreement contained in this Section 6.2 shall not apply to amounts paid in
settlement of any Claim if such settlement is effected without the prior written
consent of such Selling Shareholder, which consent shall not be unreasonably
withheld; provided, further, however, that such Selling Shareholder shall be
liable under this Agreement (including this Section 6.2 and Article 7) for only
that amount as does not exceed the net proceeds to such Selling Shareholder as a
result of the sale of Registrable Securities pursuant to such Registration
Statement. Such indemnity shall remain in full force and effect regardless of
any investigation made by or on behalf of such Indemnified Party and shall
survive the transfer of the Registrable Securities by such Selling Shareholder
pursuant to 10.7. Notwithstanding anything to the contrary contained herein, the
indemnification agreement contained in this Section 6.2 with respect to any
preliminary prospectus shall not inure to the benefit of any Indemnified Party
if the untrue statement or omission of material fact contained in the
preliminary prospectus was corrected on a timely basis in the prospectus, as
then amended or supplemented, and the Indemnified Party was promptly advised in
writing not to use the incorrect information prior to the use giving rise to a
Violation and such Indemnified Party, notwithstanding such advice, used it.

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         Section 6.3. Promptly after receipt by an Indemnified Party under this
Article 6 of notice of the commencement of any action (including any
governmental action), such Indemnified Party shall, if a Claim in respect
thereof is to be made against any indemnifying party under this Article 6,
deliver to the indemnifying party a written notice of the commencement thereof,
and the indemnifying party shall have the right to participate in, and, to the
extent the indemnifying party so desires, jointly with any other indemnifying
party similarly noticed, to assume control of the defense thereof with counsel
selected by the indemnifying party and reasonably satisfactory to the
Indemnified Party, as the case may be; provided, however, that an Indemnified
Party shall have the right to retain its own counsel with the fees and expenses
to be paid by the indemnifying party, if, in the reasonable opinion of counsel
retained by the indemnifying party, the representation by such counsel of the
Indemnified Party and the indemnifying party would be inappropriate due to
actual or potential differing interests between such Indemnified Party and any
other party represented by such counsel in such proceeding. The indemnifying
party shall pay for only one separate legal counsel for the Indemnified Persons
or the Indemnified Parties, as applicable, and such legal counsel shall be
selected by the Selling Shareholders, if the Selling Shareholders are entitled
to indemnification hereunder, or the Company, if the Company is entitled to
indemnification hereunder, as applicable. The failure to deliver written notice
to the indemnifying party within a reasonable time of the commencement of any
such action shall not relieve such indemnifying party of any liability to the
Indemnified Party under this Article 6, except to the extent that the
indemnifying party is actually prejudiced in its ability to defend such action.
The indemnification required by this Article 6 shall be made by periodic
payments of the amount thereof during the course of the investigation or
defense, as such expense, loss, damage or liability is incurred and is due and
payable.

                                  ARTICLE VII.
                                  CONTRIBUTION

         To the extent any indemnification by an indemnifying party is
prohibited or limited by law, the indemnifying party agrees to make the maximum
contribution with respect to any amounts for which it would otherwise be liable
under Article 6 to the fullest extent permitted by law; provided, however, that
(i) no contribution shall be made under circumstances where the indemnifying
party would not have been liable for indemnification under the fault standards
set forth in Article 6, (ii) no seller of Registrable Securities guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the 1933
Act) shall be entitled to contribution from any seller of Registrable Securities
who was not guilty of such fraudulent misrepresentation, and (iii) contribution
(together with any indemnification or other obligations under this Agreement) by
any seller of Registrable Securities shall be limited in amount to the net
amount of proceeds received by such seller from the sale of such Registrable
Securities.

                                 ARTICLE VIII.
                                    REPORTS

         With a view to making available to the Selling Shareholders the
benefits of Rule 144 promulgated under the 1933 Act or any other similar rule or
regulation of the SEC that may at any time permit the Selling Shareholders to
sell securities of the Company to the public without registration ("RULE 144"),
the Company agrees to:

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                  (a)      make and keep public information available, as those
         terms are understood and defined in Rule 144;

                  (b)      file with the SEC in a timely manner all reports and
         other documents required of the Company under the 1933 Act and the 1934
         Act so long as the Company remains subject to such requirements and the
         filing of such reports and other documents is required for the
         applicable provisions of Rule 144; and

                  (c)      furnish to each Selling Shareholder so long as such
         Selling Shareholder owns Registrable Securities, promptly upon request,
         such information as may be reasonably requested to permit such Selling
         Shareholder to sell such securities pursuant to Rule 144 without
         registration.

                                  ARTICLE IX.
                        AMENDMENT OF REGISTRATION RIGHTS

         Provisions of this Agreement may be amended and the observance thereof
may be waived (either generally or in a particular instance and either
retroactively or prospectively), only with written consent of the Company and
the Selling Shareholders.

                                   ARTICLE X.
                                 MISCELLANEOUS

         Section 10.1. A person or entity is deemed to be a holder of
Registrable Securities whenever such person or entity owns of record such
Registrable Securities. If the Company receives conflicting instructions,
notices or elections from two or more persons or entities with respect to the
same Registrable Securities, the Company shall act upon the basis of
instructions, notice or election received from the registered owner of such
Registrable Securities.

         Section 10.2. Any notice, demand or communication required or permitted
to be given by any provision of this Agreement must be in writing and will be
deemed to have been given and received when delivered personally or by
telefacsimile to the party designated to receive such notice, or on the date
following the day sent by overnight courier, or on the third (3rd) business day
after the same is sent by certified mail, postage and charges prepaid, directed
to the following addresses or to such other or additional addresses as any party
might designate by written notice to the other parties (or in the case of
permitted assigns, below such party's signature on the signature page to this
Agreement):

                           If to the Company:

                           Zix Corporation
                           2711 North Haskell Avenue
                           Suite 2300 LB36
                           Dallas, Texas  75246
                           Attention: General Counsel
                           Facsimile: (214) 515-7538

                                       11
<PAGE>

                           If to Aventis Holdings:

                           Aventis Holdings Inc.
                           3711 Kennett Pike
                           Greenville, DE  19807
                           Attn:  President
                           Facsimile:

                           With a copy to:

                           Aventis Pharmaceuticals Inc.
                           300 Somerset Corporate Blvd.
                           P.O. Box 6890, SC3-830A
                           Bridgewater, NJ  08807
                           Attention:  General Counsel
                           Facsimile No.:  (908) 243-7219

                           If to Aventis:

                           Aventis Inc.
                           300 Somerset Corporate Blvd.
                           P.O. Box 6890, SC3-830A
                           Bridgewater, NJ  08807
                           Attention:  General Counsel
                           Facsimile No.:  (908) 243-7219

                           With a copy to:

                           Aventis Pharmaceuticals Inc.
                           300 Somerset Corporate Blvd.
                           P.O. Box 6890, SC3-830A
                           Bridgewater, NJ  08807
                           Attn:  General Counsel
                           Facsimile:  (908) 243-7219

         Section 10.3. Failure of any party to exercise any right or remedy
under this Agreement or otherwise, or delay by a party in exercising such right
or remedy, shall not operate as a waiver thereof.

         Section 10.4. THIS AGREEMENT WILL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS WITHOUT REFERENCE TO THE
PRINCIPLES OF CONFLICT OF LAWS OR ANY OTHER PRINCIPLE THAT COULD RESULT IN THE
APPLICATION OF THE LAWS OF ANY OTHER JURISDICTION.

         Section 10.5. In the event of a breach by the Company or by a Selling
Shareholder, of any of their obligations hereunder, each Selling Shareholder or
the Company, as the case may be, in addition to being entitled to exercise all
rights granted by law and under this Agreement,

                                       12
<PAGE>

including recovery of damages, will be entitled to specific performance of its
rights under this Agreement. The Company and the Selling Shareholders agree that
monetary damages would not provide adequate compensation for any losses incurred
by reason of a breach by it of any of the provisions of this Agreement and
hereby further agrees that, in the event of any action for specific performance
in respect of such breach, it shall waive the defense that remedy at law would
be adequate. Notwithstanding anything to the contrary herein, if a Selling
Shareholder elects to receive the liquidated damages as set forth in Section
3.15 hereof, then such Selling Shareholder shall not be entitled to seek
monetary damages for such breach.

         Section 10.6. This Agreement, the Note, the Warrant, the Services
Agreement and the Asset Purchase Agreement (including all schedules and exhibits
thereto) constitute the entire agreement among the parties hereto with respect
to the subject matter hereof and thereof. There are no restrictions, promises,
warranties or undertakings, other than those set forth or referred to herein and
therein. This Agreement, the Note, the Warrant, the Services Agreement and the
Asset Purchase Agreement supersede all prior agreements and understandings among
the parties hereto with respect to the subject matter hereof and thereof.

         Section 10.7. Each Selling Shareholder may assign its rights hereunder
to any assignee of the Registrable Securities provided that such assignee
executes a joinder page indicating its agreement to be bound to the terms and
conditions of this Agreement. This Agreement shall inure to the benefit of and
be binding upon the successors and permitted assigns of each of the parties
hereto.

         Section 10.8. The headings in this Agreement are for convenience of
reference only and shall not limit or otherwise affect the meaning hereof.

         Section 10.9. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original but all of which shall
constitute one and the same agreement. This Agreement, once executed by a party,
may be delivered to the other parties hereto by facsimile transmission of a copy
of this Agreement bearing the signature of the party so delivering this
Agreement.

         Section 10.10. Each party shall do and perform, or cause to be done and
performed, all such further acts and things, and shall execute and deliver all
such other agreements, certificates, instruments and documents, as the other
parties may reasonably request in order to carry out the intent and accomplish
the purposes of this Agreement and the consummation of the transactions
contemplated hereby.

         Section 10.11. The language used in this Agreement will be deemed to be
the language chosen by the parties to express their mutual intent, and no rules
of strict construction will be applied against any party.

         Section 10.12. In the event that any provision of this Agreement is
invalid or unenforceable under any applicable statute or rule of law, then such
provision shall be deemed inoperative to the extent that it may conflict
therewith and shall be deemed modified to conform with such statute or rule of
law. Any provision hereof which may prove invalid or unenforceable under any law
shall not affect the validity or enforceability of any other provision hereof.

                                       13
<PAGE>

         IN WITNESS WHEREOF, the Company and each Selling Shareholder have
caused this Agreement to be duly executed as of the date first above written.

                                  ZIX CORPORATION

                                  By: /s/ Ronald A. Woessner
                                      ----------------------------------------
                                      Name:  Ronald A. Woessner
                                      Title: Senior Vice President

                                  AVENTIS INC.

                                  By: /s/ Juergen Lasowski
                                      ----------------------------------------
                                      Name:  Juergen Lasowski
                                      Title: Authorized Signatory

                                  AVENTIS HOLDINGS INC.

                                  By: /s/ Juergen Lasowski
                                      ----------------------------------------
                                      Name:  Juergen Lasowski
                                      Title: Authorized Signatory

                 SIGNATURE PAGE TO REGISTRATION RIGHTS AGREEMENT

<PAGE>

                                     JOINDER

The undersigned hereby executes this signature page to that certain Registration
Rights Agreement (the "Registration Rights Agreement"), dated January 30, 2004,
between Aventis Inc., Aventis Holdings Inc. and Zix Corporation, thereby
indicating the undersigned's agreement to the terms and conditions of the
Registration Rights Agreement, as fully and effectively as if the undersigned
had executed and delivered this page at the time originally executed by each of
Aventis Inc., Aventis Holdings Inc. and Zix Corporation.

                                         By: __________________________________
                                             Name:
                                             Title:<PAGE>

                                                                     EXHIBIT 4.3

                               SECURITY AGREEMENT

         This SECURITY AGREEMENT is made and entered into as of January 30, 2004
by and between Aventis Inc., a Pennsylvania corporation ("Secured Party"), and
Zix Corporation, a Texas corporation ("Debtor").

         WHEREAS, Debtor has delivered a Promissory Note dated as of the date
hereof in the original principal amount of $3,000,000 payable to Secured Party
(the "Note");

         WHEREAS, Debtor and Secured Party have entered into a Master Services
Agreement dated as of the date hereof (the "Master Services Agreement"),
pursuant to which Secured Party paid $4,000,000 to Debtor in respect of future
services to be performed by Debtor thereunder;

         WHEREAS, in connection with the delivery of the Note and the Master
Services Agreement, the parties desire to enter into this Agreement to secure
the obligations of Debtor to Secured Party under the Note and the Master
Services Agreement; and

         WHEREAS, any capitalized term used but not otherwise defined herein
shall have the meaning ascribed to such term in the Uniform Commercial Code of
the State of Texas (the "UCC"), as such law is in effect on the date hereof.

         NOW, THEREFORE, in consideration of the foregoing premises and the
mutual covenants and agreements set forth herein, and for other good and
valuable consideration, the mutuality, receipt and sufficiency of which are
hereby acknowledged, and intending to be legally bound, the parties hereto agree
as follows:

1.       Security Interest. Debtor hereby grants, assigns, pledges and transfers
to Secured Party and grants Secured Party a first priority continuing security
interest in the Collateral, whether now owned or existing, or hereafter acquired
or arising, or in which Debtor may have an interest, and wheresoever located.
"Collateral" means (a) all of Debtor's Accounts, Equipment and Fixtures and (b)
any proceeds relating to the foregoing.

2.       Obligations Secured. The Collateral secures (i) the payment in full,
promptly when due, of any and all indebtedness (including principal and
interest), and any other obligations of the Debtor to the Secured Party under
the Note and (ii) the Prepaid Amount (as such term is defined in the Master
Services Agreement) under the Master Services Agreement.

3.       Maintenance of Collateral. The Debtor will maintain the Equipment and
Fixtures necessary in the operation of its business in good condition and
repair, ordinary wear and tear excepted.

4.       Insurance of Collateral. The Debtor will insure itself and the
Collateral against such perils and to such limits as are ordinarily carried by
similar businesses.

<PAGE>

5.       Access to Books and Records. The Debtor agrees that the Secured Party
may at reasonable times and on three business days' notice, have adequate and
convenient access to the books and records of the Debtor related to the
Collateral at the place where they may be located for the purpose of examining,
auditing or copying the same.

6.       Defaults and Remedies.

         (a)      If (i) an Event of Default (as such term is defined in the
Note) shall occur or (ii) if Secured Party shall terminate the Master Services
Agreement pursuant to Section 4.2(b), 4.3, 4.5 (as a result of the insolvency or
similar occurrence of Debtor) or 14.1 of the Master Services Agreement, after
expiration of requisite notice and cure periods as set forth therein (the
occurrence of either (i) or (ii) is referred to herein as a "Default"), then
Secured Party shall have, in addition to all other rights set forth in the Note,
the Master Services Agreement, or provided herein or by law, the rights and
remedies of a secured party under the UCC. In addition to all other sums due the
Secured Party hereunder, the Debtor shall pay the Secured Party all costs and
out-of-pocket expenses incurred by the Secured Party, including reasonable
attorneys' fees and court costs, in obtaining, liquidating or enforcing payment
of Collateral or the obligations of the Debtor under the Note (collectively, the
"Obligations") or in the prosecution or defense of any action or proceeding by
or against the Secured Party or Debtor concerning any matter arising out of or
connected with this Agreement or the Collateral or the Obligations. Any
requirement of reasonable notice shall be met if such notice is personally
served on or mailed, postage prepaid, to Debtor in accordance with Section 11(b)
hereof at least 10 days before the time of sale or other event giving rise to
the requirement of such notice; provided; however, no notification need be given
to Debtor if Debtor has signed, after a Default has occurred, a statement
renouncing any right to notification of sale or other intended disposition. The
Secured Party shall not be obligated to make any sale or other disposition of
the Collateral regardless of notice having been given. The Secured Party may be
the purchaser at any such sale. The Secured Party may postpone or cause the
postponement of the sale of all or any portion of the Collateral by announcement
at the time and place of such sale, and such sale may, without further notice,
be made at the time and place to which the sale was postponed or the Secured
Party may further postpone such sale by announcement made at such time and
place.

         (b)      Without in any way limiting the foregoing, upon the occurrence
and during the continuation of any Default, the Secured Party shall have the
right, in addition to all other rights set forth in the Note, the Master
Services Agreement, or provided herein or by law, to take physical possession of
any and all of the Collateral and anything found therein, the right for that
purpose to enter without legal process any premises where the Collateral may be
found (provided such entry be done lawfully), and the right to maintain such
possession on Debtor's premises (Debtor hereby agreeing to lease such premises
without cost or expense to the Secured Party or its designee if the Secured
Party so requests) or to remove the Collateral or any part thereof to such other
places as the Secured Party may desire. Upon the occurrence and during the
continuation of any Default, Debtor shall, upon the Secured Party's demand,
assemble the Collateral and make it available to the Secured Party at a place
designated by the Secured Party.

7.       Further Assurances; Immunities of Secured Party and Secured Party, etc.
With respect to the Collateral and any security interest or lien therein, the
Debtor agrees, promptly upon request of the Secured Party, to do, file, record,
make, execute and deliver all such acts, deeds, things,

                                        2
<PAGE>

notices and instruments as may be reasonably necessary in the opinion of the
Secured Party in order to effect the liens on and security interests in the
Collateral created hereby and the enforcement and realization of the benefits
of, all of the rights, remedies and powers of the Secured Party hereunder
relating to the Collateral.

8.       Transfer of Interest. The Debtor will not, except with the written
consent of the Secured Party, sell, assign, transfer or otherwise dispose of any
of the Collateral except (a) in the ordinary course of business, (b) assets
which are the subject of a casualty or a condemnation (including any deed in
lieu thereof), (c) dispositions of Accounts in connection with the collection or
compromise thereof, (d) dispositions of assets with an aggregate fair market
value of less than $1,000,000, for less than 75% of the fair market value of
such assets and (e) any other disposition so long as the Debtor uses proceeds of
such disposition to prepay the Note in accordance with the terms thereof.

9.       Expenses. The Debtor will pay all costs of filing of any financing,
continuation or termination statements with respect to the Collateral deemed by
the Secured Party to be reasonably necessary or advisable in order to perfect
and protect the liens and security interests hereby created in favor of the
Secured Party or intended so to be. The Debtor agrees that the Collateral
secures, and further agrees to pay on demand, all reasonable expenses (including
but not limited to reasonable attorneys' fees and other costs for legal
services, costs of insurance and payments of taxes or other charges) of or
incidental to the custody, care, sale or collection of or realization on any of
the Collateral or in any way relating to the enforcement or protection of the
rights of the Secured Party hereunder.

10.      Continuing Agreement. This Agreement shall be a continuing agreement in
every respect and shall remain in full force and effect until the Obligations
have been fully paid or performed. Upon such termination of this Agreement, the
Secured Party shall, upon the request of Debtor, forthwith release its security
interests hereunder.

11.      General.

         (a)      This Agreement cannot be modified or amended except in writing
signed by both parties. All of the rights, privileges, remedies, and options
given to the Secured Party hereunder shall inure to the benefit of its
successors and assigns, and all the terms, conditions, covenants, agreements,
representations, and warranties of and in this Agreement shall bind the Debtor
and its legal representatives, successors and assigns, provided that Debtor may
not assign its rights or delegate its duties hereunder without the Secured
Party's prior written consent.

         (b)      All notices, requests, demands and other communications with
respect hereto shall be given to the parties hereto in accordance with the
notice provisions contained in the Purchase Agreement.

         (c)      In the event and to the extent that any provision hereof shall
be deemed to be invalid or unenforceable by reason of the operation of any law
or by reason of the interpretation placed thereon by any court, this Agreement
shall to such extent be construed as not containing such provision, but only as
to such locations where such law or interpretation is operative, and the
invalidity or unenforceability of such provision shall not affect the validity
of any remaining

                                       3
<PAGE>

provisions hereof, and any and all other provisions hereof which are otherwise
lawful and valid shall remain in full force and effect.

         (d)      This Agreement, the Note and the Master Services Agreement
represent the entire agreement between the parties and shall supersede all prior
agreements, commitments and writings with respect to the subject hereof.

         (e)      This Agreement shall be deemed to have been made in the State
of Texas and shall be governed by, and construed in accordance with, the laws of
the State of Texas. The headings in this Agreement are for convenience of
reference only and shall not limit or otherwise affect the meaning of any
provision hereof.

         (f)      This Agreement may be executed in any number of counterparts
and by different parties hereto on separate counterpart signature pages, each
constituting an original, but all together one and the same instrument.

                            [SIGNATURE PAGE FOLLOWS]

                                       4
<PAGE>

         IN WITNESS WHEREOF, the parties have caused this Security Agreement to
be duly executed and delivered as of the date and year first above written.

                                 ZIX CORPORATION

                                 By: /s/ Ronald A. Woessner
                                     -----------------------------------
                                     Name:  Ronald A. Woessner
                                     Title: Senior Vice President

                                 AVENTIS INC.

                                 By: /s/ Juergen Lasowski
                                     -----------------------------------
                                     Name:  Juergen Lasowski
                                     Title: Authorized Signatory

                      SIGNATURE PAGE TO SECURITY AGREEMENT

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