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                                                                   EXHIBIT 10.15

                                 ZIX CORPORATION
                             STOCK OPTION AGREEMENT

Date of Grant                                                    Expiration Date

February 24, 2004                                              February 23, 2014

         THIS STOCK OPTION AGREEMENT (this "Option Agreement") is made and
entered into as of the 24th day of February 2004 by and between Zix Corporation,
a Texas corporation (the "Company"), and Richard Spurr ("Optionee").

         WHEREAS, the Company desires to employ Optionee as President and Chief
Operating Officer of the Company, and Optionee desires to be employed by the
Company in such capacities pursuant to an Employment Agreement, dated as of
January 20th, 2004 (the "Employment Agreement");

         WHEREAS, pursuant to the terms of the Employment Agreement, the Company
has agreed to grant stock options to Optionee to purchase 650,000 shares of the
Common Stock, par value $.01 per share of the Company (the "Common Stock");

         NOW, THEREFORE, the parties intending to be legally bound, agree as
follows:

1.       OPTION GRANT. In consideration of the mutual agreements and covenants
contained herein and in the Employment Agreement, the Company hereby grants to
the Optionee, on the terms and conditions and subject to the restrictions as set
forth in this Option Agreement, a non-qualified stock option ("Option"), to
purchase 650,000 shares of Common Stock at a price per share (the "Option
Price") equal to $10.80.

2.       DEFINITIONS.

         a.       Acquiring Person. An "Acquiring Person" shall mean any person
(including any "person" as such term is used in Sections 13 (d) (3) or 14 (d)
(2) of the Exchange Act) that, together with all Affiliates and Associates of
such person, is the beneficial owner of 35% or more of the outstanding Common
Stock. The term "Acquiring Person" shall not include the Company, any subsidiary
of the Company, any employee benefit plan of the Company (or trust with respect
thereto) or subsidiary of the Company, or any person holding Common Stock of the
Company for or pursuant to the terms of any such plan. For the purposes of this
Option Agreement, a person who becomes an Acquiring Person by acquiring
beneficial ownership of 35% or more of the Common Stock at any time after the
date of this Option Agreement shall continue to be an Acquiring Person whether
or not such person continues to be the beneficial owner of 35% or more of the
outstanding Common Stock.

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         b.       Affiliate and Associate. "Affiliate" and "Associate" shall
have the respective meanings ascribed to such terms in Rule 12b-2 of the General
Rules and Regulations under the Exchange Act in effect on the date of this
Option Agreement.

         c.       Board. "Board" shall mean the Board of Directors of the
Company.

         d.       Change in Control. A "Change in Control" of the Company shall
have occurred if at any time during the term of this Option Agreement any of the
following events shall occur:

                  (i)      Any Sale of the Company or any Material Subsidiary;
         or

                  (ii)     Any Acquiring Person has become the beneficial owner
         of securities which, when added to any securities already owned by such
         person, would represent in the aggregate 35% or more of the
         then-outstanding securities of the Company that are entitled to vote to
         elect any class of directors;

                  (iii)    If, at any time, the Continuing Directors then
         serving on the Board cease for any reason to constitute at least a
         majority thereof; or

                  (iv)     Any occurrence that would be required to be reported
         in response to Item 6(e) of Schedule 14A of Regulation 14A or any
         successor rule or regulation promulgated under the Exchange Act.

         e.       Continuing Director. A "Continuing Director" shall mean a
director of the Company who (i) is not an Acquiring Person or an Affiliate or
Associate thereof, or a representative of an Acquiring Person or nominated for
election by an Acquiring Person, and (ii) was either a member of the Board on
the date of this Option Agreement or subsequently became a director of the
Company and whose initial election or initial nomination for election by the
Company's shareholders was approved by a majority of the Continuing Directors
then on the Board.

         f.       Exchange Act. "Exchange Act" shall mean the Securities
Exchange Act of 1934, as amended, and the rules and regulations promulgated
thereunder.

         g.       Fair Market Value. "Fair Market Value" shall mean the closing
sale price (or average of the quoted closing bid and asked prices if there is no
closing sale price reported) of the Common Stock on the date specified as
reported by the Nasdaq National Market, or by the principal national stock
exchange on which the Common Stock is then listed. If there is no reported price
information for such date, the Fair Market Value will be determined by the
reported price information for Common Stock on the day nearest preceding such
date.

         h.       Material Subsidiary. A "Material Subsidiary" shall mean any
majority-owned subsidiary of the Company that is material to the business of the
Company, taken as a whole.

         i.       Person. A "Person" shall mean an individual, a corporation, a
partnership, an association, a joint-stock company, a trust, an incorporated
organization, or a government or

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political subdivision thereof and any other entity. A Person, together with that
Person's Affiliates and Associates, and any Persons acting as a partnership,
limited partnership, joint venture, association, syndicate, or other group
(whether or not formally organized), or otherwise acting jointly or in concert
or in a coordinated or consciously parallel manner (whether or not pursuant to
any express agreement), for the purpose of acquiring, holding, voting, or
disposing of securities of the Company with that Person, shall be deemed a
single "Person."

         j.       Sale. A "Sale" occurs with respect to the Company or a
Material Subsidiary, as applicable, if it engages in a merger, consolidation,
recapitalization, reorganization, or sale, lease, license, transfer, or other
effective disposition of all or substantially all of the Company's or Material
Subsidiary's assets and the Company or its shareholders or Affiliates
immediately before such transaction beneficially own, immediately after or as a
result of such transaction, equity securities of the surviving or acquiring
corporation or such corporation's parent corporation possessing less than 51% of
the voting power of the surviving or acquiring Person or such Person's parent
corporation, provided that a Sale shall not be deemed to occur upon any public
offering or series of such offerings of securities of the Company or a Material
Subsidiary that results in any such change in beneficial ownership.

         k.       Securities Act. "Securities Act" shall mean the Securities Act
of 1933, as amended, and the rules and regulations promulgated thereunder.

3.       TERM OF OPTION AND VESTING.

         a.       The term of this Option shall expire, and shall not be
exercisable with respect to any vested shares of Common Stock hereunder as to
which the Option has not been exercised, on the tenth anniversary of the date
hereof as set forth under "Expiration Date" above (the "stated term"), except as
such stated term may be otherwise shortened by the other provisions set forth in
Paragraph 5 below. Except as otherwise provided in subparagraph (b) below, the
Option shall vest in increments as follows:

<TABLE>
<CAPTION>
     Date                      Shares Vesting        Aggregate Shares Vested
     ----                      --------------        -----------------------
<S>                            <C>                   <C>
April 20, 2004                    162,500                   162,500
July 20, 2004                      44,318                   206,818
October 20, 2004                   44,318                   251,136
January 20, 2005                   44,318                   295,454
April 20, 2005                     44,318                   339,772
July 20, 2005                      44,318                   384,090
October 20, 2005                   44,318                   428,408
January 20, 2006                   44,318                   472,726
April 20, 2006                     44,318                   517,044
July 20, 2006                      44,318                   561,362
October 20, 2006                   44,319                   605,681
January 20, 2007                   44,319                   650,000
</TABLE>

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         b.       Upon the occurrence of: a Change in Control, the Option shall
immediately vest in full and remain exercisable for a period of six months as to
all shares of Common Stock subject thereto. Furthermore, if Company terminates
Employee's employment pursuant to Section 9(E)(without "Cause") of the
Employment Agreement or Employee resigns employment pursuant to Section
9(D)(1)(with "Good Reason") of the Employment Agreement, then the option shares
under this Option that would have become vested on the next vesting date (had
Employee still been employed on the next vesting date) shall become vested as of
the Date of Termination (as defined in the Employment Agreement); and Optionee
shall have the period specified in Paragraph 5.a. below to exercise this Option
with respect to the vested option shares.

4.       EXERCISE OF OPTION.

         a.       The Option shall be exercisable only with respect to vested
shares of Common Stock subject thereto and subject to Paragraph 5 below, shall
not be exercisable with respect to any vested shares of Common Stock unless
Optionee shall, at the time of exercise, be an employee, consultant or director
of the Company or a Material Subsidiary, except as covered in Paragraph 3(b)
above or Paragraph 5 below. However, this Option may not be exercised as to less
than 100 vested shares at any one time (or the remaining shares then purchasable
under this Option, if less than 100 shares).

         b.       This Option may be exercised with respect to any vested shares
of Common Stock only by written notice (the "Exercise Notice") by Optionee to
the Company at its principal executive office to the attention of the Company's
chief financial officer. The Exercise Notice shall be deemed given when
deposited in the U. S. mails, postage prepaid, addressed to the Company at its
principal executive office, or if given other than by deposit in the U.S. mails,
when delivered in person to an executive officer of the Company at that office.
The date of exercise of this Option shall be the date of the postmark if the
notice is mailed or the date received if the notice is delivered other than by
mail. The Exercise Notice shall state the number of shares in respect of which
this Option is being exercised and, if the shares for which this Option is being
exercised are to be evidenced by more than one stock certificate, the
denominations in which the stock certificates are to be issued. The Exercise
Notice shall be signed by Optionee and shall include his complete address,
together with his social security number.

         c.       This Option may be exercised either by tendering cash in the
amount of the Option Price or, with the Company's consent, such consent to be
given by the Compensation and Stock Option Committee of the Board, subject to
compliance with applicable requirements of Section 16(b) under the Exchange Act,
by tendering shares of Common Stock. The Exercise Notice shall be accompanied by
payment of the aggregate Option Price of the shares purchased by cash, a
certified cashier's check or, at the Company's option, by delivery of shares of
Common Stock having a Fair Market Value on the date immediately preceding the
exercise date equal to the Option Price. The Exercise Notice shall also be
accompanied by payment of the amount that the Company is required to withhold
for federal income or other tax purposes (unless the Optionee has provided for
payment of those taxes to the Company in another manner permitted under this
Option Agreement). If the Option is exercised in full or in part, the Optionee
shall surrender this Option Agreement to the Company so that the Company may
make

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appropriate notation hereon or cancel this Option Agreement and issue a new
agreement representing the unexercised portion of the Option.

         d.       If the shares of Common Stock issued upon the exercise of the
Option are covered by an effective registration statement under the Securities
Act, the Option may be exercised by a broker-dealer acting on behalf of the
Optionee if (i) the broker-dealer has received from the Optionee or the Company
a fully- and duly-endorsed agreement evidencing the Option, together with
instructions signed by the Optionee requesting the Company to deliver the shares
of Common Stock subject to the Option to the broker-dealer on behalf of the
Optionee and specifying the account into which such shares should be deposited,
(ii) adequate provision has been made with respect to the payment of any
withholding taxes due upon such exercise, (iii) the broker-dealer delivers to
the Company the aggregate Option Price in accordance with the paragraph above,
and (iv) the broker-dealer and the Optionee have otherwise complied with Section
220.3(e)(4) of Regulation T, 12 CFR Part 220, or any successor provision.

         e.       The certificates for shares of Common Stock as to which this
Option shall have been so exercised shall be registered in the name of the
Optionee and shall be delivered to the Optionee at the address specified in the
Exercise Notice. An option exercise shall be valid only if the Optionee makes
payment or other arrangements relating to the withholding tax obligations
discussed in Paragraph 9. In the event the person exercising this Option is a
transferee of the Optionee by will or under the laws of descent and
distribution, the Exercise Notice shall be accompanied by appropriate proof of
the right of such transferee to exercise this Option.

         f.       This Option is not transferable otherwise than by will or the
laws of descent and distribution, and is exercisable during the Optionee's
lifetime only by Optionee. Without limiting the generality of the foregoing,
this Option may not be assigned, transferred (except as aforesaid), pledged or
hypothecated in any way (whether by operation of law or otherwise), and shall
not be subject to execution, attachment, or similar process, without the prior
written consent of the Company. Any attempted assignment, transfer, pledge, or
hypothecation contrary to the provisions hereof shall be void and ineffective
for all purposes.

5.       TERMINATION OF OPTION.

         a.       In the event Optionee ceases to be an employee of either the
Company or a Material Subsidiary in the circumstances set forth below, this
Option may be exercised with by the Optionee or his estate, personal
representative or beneficiary to the fullest extent that Optionee was entitled
to exercise the same on the Date of Termination (as defined in the Employment
Agreement), after giving effect to the provisions of Paragraph 3.b. above, for
the periods specified below:

<TABLE>
<S>                                                     <C>
Termination pursuant to Section 9(A) (death)            One year from death

Termination pursuant to Section 9(B) (by Company for    Six months from separation date
disability)

Termination pursuant to Section 9(C) (by Company for    Option shall automatically expire simultaneously
"Cause")                                                with separation date
</TABLE>

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<TABLE>
<S>                                                     <C>
Termination pursuant to Section 9(D)(1) (by Optionee    Six months from separation date
for "Good Reason")

Termination pursuant to Section 9(D)(2) (by Employee    Six months from separation date
for health reasons)

Termination pursuant to Section 9(E)(by Company         One year from separation date
without "Cause")

Termination pursuant to Section 9(F) (by Employee       30 days from separation date
other than for Good Reason or health impairment)
</TABLE>

         b.       After the Optionee's death, this Option shall be exercisable
only by the executor or administrator of the Optionee's estate, or if the
Optionee's estate is not in administration, by the person or persons to whom the
Optionee's rights shall have passed by the Optionee's will or under the laws of
descent and distribution of the state where the Optionee was domiciled at the
date of death.

6.       RIGHTS AS SHAREHOLDER. Neither the Optionee nor any person claiming
under or through the Optionee shall be or have any rights or privileges of a
shareholder of the Company in respect of any of the shares of Common Stock
issuable upon the exercise of this Option, unless and until certificates
representing such shares of Common Stock shall have been issued (as evidenced by
the appropriate entry on the books of the Company or of a duly authorized
transfer agent of the Company).

7.       STATE AND FEDERAL SECURITIES REGULATION.

         a.       The shares of Common covered by the Option have not been
registered under the Securities Act or qualified under applicable state
securities laws. The Company agrees to use commercially reasonable efforts to
register the shares of Common Stock covered by the Option on a registration
statement on Form S-8 within 90 days after the date of this Option Agreement and
to use commercially reasonable efforts to maintain the effectiveness of such
registration statement during the term of this Option Agreement and any period
during which this Option is exercisable, subject to the following provisions
related to the Company's ability to temporarily suspend the effectiveness of
such registration statement.

         b.       No shares of Common Stock covered by this Option shall be
issued by the Company upon the exercise of this Option unless and until any
then-applicable requirements of state and federal laws and regulatory agencies
shall have been fully complied with to the satisfaction of the Company and its
counsel. The Company may suspend for a reasonable period or periods the time
during which this Option may be exercised if, in the opinion of the Company,
such suspension is required to enable the Company to remain in compliance with
regulatory requirements relating to the issuance of shares of Common Stock
subject to this Option. This Option is subject to the requirement that, if at
any time the Company shall determine, in its discretion, that the listing,
registration or qualification of the shares of Common Stock subject to this
Option upon any securities exchange or under any state or federal law, or the
consent or approval of any government regulatory body, is necessary or desirable
as a condition of, or in connection with, the granting or exercise of this
Option or the issue or purchase of shares under

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this Option, this Option may not be exercised in whole or in part until such
listing, registration, qualification, consent or approval shall have been
effected or obtained free of any conditions not acceptable to the Company. The
Company shall be under no obligation to effect or obtain any such listing,
registration, qualification, consent or approval if the Company shall determine,
in its discretion, that such action would not be in the best interest of the
Company. The Company shall not be liable for damages due to a delay in the
delivery or issuance of any stock certificates for any reason whatsoever,
including, but not limited to, a delay caused by listing, registration or
qualification of the shares of Common Stock subject to an option upon any
securities exchange or under any federal or state law or the effecting or
obtaining of any consent or approval of any governmental body with respect to
the granting or exercise of this Option or the issue or purchase of shares under
this Option. At the Company's election, the certificate evidencing shares of
Common Stock issued to the Optionee may be legended as follows, if necessary:

         THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
         UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER THE APPLICABLE
         SECURITIES LAWS OF ANY STATE OR OTHER JURISDICTION AND MAY NOT BE
         OFFERED FOR SALE, SOLD, ASSIGNED, TRANSFERRED, PLEDGED, OR OTHERWISE
         DISPOSED OF EXCEPT IN COMPLIANCE WITH THE REQUIREMENTS OF SUCH ACT AND
         THE APPLICABLE SECURITIES LAWS OF ANY STATE OR OTHER JURISDICTION.

Also, if applicable, a legend evidencing any transfer restrictions imposed by
this Option Agreement may also, at the Company's election, be affixed to the
shares of Common Stock issued to the Optionee.

8.       ADJUSTMENT. If there is any change in the number or class of shares of
Common Stock through the declaration of stock or cash dividends, or
recapitalization resulting in stock splits, or combinations or exchanges of such
shares, the number or class of shares subject to the Option and/or the Option
Price may be proportionately adjusted by the Board in its sole discretion to
reflect any such change in the number or class of issued shares of Common Stock;
provided, however, that any fractional shares resulting from any such adjustment
shall be eliminated. In the event of any other extraordinary corporate
transaction, including but not limited to, distributions of cash or other
property to the Company's shareholders, the Board may equitably adjust the
shares subject to the Option and/or the Option Price as it deems appropriate in
its sole discretion.

9.       WITHHOLDING OF TAXES. The Company may make such provisions and take
such steps as it may deem necessary or appropriate for the withholding of any
taxes which the Company or any subsidiary is required by any law or regulation
of any governmental authority, whether federal, state or local, domestic or
foreign, to withhold all or any portion of the shares of Common Stock subject to
this Option until the Optionee reimburses the Company or the applicable
subsidiary for the amount the Company or the applicable subsidiary is required
to withhold with respect to such taxes, subject to compliance with applicable
requirements of Section 16(b) under the Exchange Act, canceling, retaining or
withholding any portion of such shares of Common Stock in an amount sufficient
to reimburse the Company or the applicable

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subsidiary for the minimum amount it is required to so withhold, or taking any
other action reasonably required to satisfy the minimum withholding obligation
of the Company or the applicable subsidiary.

10.      CONTINUED EMPLOYMENT NOT PRESUMED. Nothing in this Option Agreement or
any document describing it nor the grant of the Option shall give the Optionee
the right to continue in employment with the Company or any of its Subsidiaries
or affect the right of the Company to terminate the employment of the Optionee
with or without Cause (as defined in the Employment Agreement).

11.      APPLICABILITY OF 2003 NEW EMPLOYEE PLAN. This Option is not issued
pursuant to any formal stock option plan of Zix Corporation. However, the
applicable provisions of the Zix Corporation 2003 New Employee Stock Option Plan
(relating to the administrative matters) (referred to herein as the "Plan") are
incorporated herein by reference. In the event of any inconsistency, the
provisions of the Plan shall govern, provided that no amendment shall be made to
the Plan subsequent to the date hereof that impairs the Optionee's rights under
this Option without the Optionee's written consent. The option shares covered by
this agreement do not count toward the number of shares issuable under the Plan.

12.      GOVERNING LAW. The interpretation, performance and enforcement of this
Option Agreement shall be governed by and construed in accordance with the laws
of the State of Texas and the United States, as applicable, without reference to
the conflict of laws rules thereof.

13.      ENTIRE AGREEMENT. This Option Agreement, the Employment Agreement, and
the Plan (to the extent applicable) constitute the entire agreement between the
parties pertaining to the subject matter hereof and supersede all prior and
contemporaneous agreements, representations and understandings of the parties.
No supplement, modification or amendment of this Option Agreement shall be
binding unless executed in writing by the party to be charged therewith. No
waiver of any of the provisions of this Option Agreement shall be deemed, or
shall constitute a waiver of any other provision, whether or not similar, nor
shall any waiver constitute a continuing waiver.

14.      DUPLICATE ORIGINALS. Duplicate originals of this document shall be
executed by both the Company and the Optionee, each of which shall retain one
duplicate original.

15.      NOTICE. Other than any Exercise Notice, any notice required or
permitted to be given under this Option Agreement shall be in writing and
delivered in person or sent by registered or certified mail, return receipt
requested, first-class postage prepaid, (i) if to the Optionee, at the address
shown on the books and records of the Company, or (ii) if to the Company, at
2711 N. Haskell Avenue, Suite 2300, LB 36, Dallas, Texas 75204-2911: Attention:
Treasurer, or any other address that may be given by either party to the other
party by notice pursuant to this Paragraph. Any notice other than any Exercise
Notice, if sent by registered or certified mail, shall be deemed to have been
given when received.

16.      MISCELLANEOUS.

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         a.       If any provision of this Option Agreement is declared or found
to be illegal, unenforceable or void, in whole or in part, then the parties
shall be relieved of all obligations arising under such provision, but only to
the extent that it is illegal, unenforceable, or void, it being the intent and
agreement of the parties that this Option Agreement shall be deemed amended by
modifying such provision to the extent necessary to make it legal and
enforceable while preserving its intent or, if that is not possible, by
substituting therefor another provision that is legal and enforceable and
achieves the same objectives.

         b.       All paragraph and, section titles and captions in this Option
Agreement are for convenience only, shall not be deemed part of this Option
Agreement, and in no way shall define, limit, extend, or describe the scope or
intent of any provisions of this Option Agreement.

         c.       No failure by any party to insist upon the strict performance
of any covenant, duty, agreement or condition of this Option Agreement or to
exercise any right or remedy consequent upon a breach thereof shall constitute
waiver of any such breach or any other covenant, duty, agreement, or condition.

                                           THE COMPANY:

                                           ZIX CORPORATION

                                           By: /s/ John A. Ryan
                                               ---------------------------------
                                               John A. Ryan, CEO

                                           OPTIONEE:

                                           /s/ Richard Spurr
                                           -----------------
                                           Richard Spurr

                                       9

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