Document:

<PAGE>
                                                                   EXHIBIT 10.13

                                 AMENDMENT NO. 5
                   TO AMTC CORPORATION STOCK OPTION AGREEMENT

         This Agreement is entered into as of March 1, 2003, between David P.
Cook ("Cook") and Zix Corporation, a Texas corporation ("ZixCorp"). Reference is
made to that certain AMTC Corporation Stock Option Agreement, effective as of
April 29, 1998, between Cook and ZixCorp (the "Cook Option").

1.       Amendments. The Cook Option is amended as follows:

         o    Revise Section 5 in its entirety to read as follows:

              "5. Expiration of Option. The Option shall expire, and shall not
              be exercisable thereafter with respect to any vested shares of
              Common Stock hereunder as to which the Option has not been
              exercised, on the sixth anniversary of the Award Date."

         o    Delete clause (c) and renumber existing clause (d) as clause (c)
              in Section 3.

2.       No Other Changes. Other than as provided herein, the Cook Option shall
         remain in full force and effect.

         Executed on the dates set forth below, to be effective as of the date
first set forth above.

                                     ZIX CORPORATION

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

                                     Date:   3-07-03
                                          -----------------------

                                     /s/ David P. Cook
                                     ----------------------------
                                     David P. Cook

                                     Date:   2/28/2003
                                          -----------------------

                                     /s/ Cheryl G. Cook
                                     ----------------------------
                                     Cheryl G. Cook

                                     Date:   2-28-03
                                          -----------------------<PAGE>
                                                                   EXHIBIT 10.23

         THIS OFFER TO SUBLEASE ("Sublease") made the 1st day of August 2002
BETWEEN:

                   OPTIWAVE CORPORATION/CORPORATION OPTIWAVE

         (a Canadian corporation, hereinafter called the "Sublandlord" for the
purpose of this Sublease only)

         OF THE FIRST PART

AND:

                                ZIXIT CORPORATION

         (a Texas, U.S.A. corporation, hereinafter called the "Tenant" for the
         purpose of this Sublease only) OF THE SECOND PART

AND:

                      WAIDT CONSTRUCTION & DEVELOPMENTS LTD

         (hereinafter called the "Landlord")

         OF THE THIRD PART

         WHEREAS by written lease dated August 9, 2001 between WAIDT
CONSTRUCTION & DEVELOPMENTS LTD. and OPTIWAVE CORPORATION, the Sublandlord
leased a building from the Landlord located on property known municipally as 7
Capella Court, Nepean, Ontario (the "Building"), a copy of said lease with any
schedules, exhibits and amendments thereto (the "Head Lease") being attached
hereto as Exhibit A and incorporated herein by reference. All undefined
capitalized terms used in this Sublease have the meaning given to them in the
Head Lease. All currency references herein are in Canadian dollars.

         AND WHEREAS the Tenant has agreed to sublet a portion of the Building
located on the ground floor of the Building, as shown on the floor plan attached
hereto as Exhibit B and incorporated herein by reference (the "Leased
Premises").

         AND WHEREAS the Sublandlord has agreed to grant this Sublease of the
Leased Premises upon .the following terms and conditions.

         NOW THEREFORE in consideration of the rents, covenants and conditions
herein reserved and contained, the parties agree as follows:

1.       TERM:
         The Sublandlord hereby Subleases the Leased Premises to the Tenant for
         a term of One Year and One Month (the "Term") commencing on the 1st day
         of August 2002 ("the Commencement Date") and ending one minute prior to
         midnight on the 31st day of August 2003 ("the Termination Date"),
         unless renewed or terminated as hereinafter provided. The Sublandlord
         shall deliver possession of the Leased Premises to Tenant upon the full
         execution hereof. In the event the Head Lease goes into default or is
         properly terminated prior to the expiry or termination of the Sublease
         term hereby granted, including any renewals thereof, the Landlord
         shall, at its sole option, have the right by giving notice to the
         Sublandlord and to the Tenant to assume the Sublease for the remaining
         portion of the Sublease term then outstanding. The parties hereby
         consent to such an assumption of the Sublease by the Landlord only in
         the circumstances described above.

<PAGE>

2.       GROSS RENT:
         Tenant shall pay directly to the Landlord, to be applied to the
         Sublandlord's rental account, as Gross Rent for the Leased Premises and
         payable in equal monthly installments in advance on the first day of
         each calendar month during the Term of the Sublease, excluding the
         first month of the Term being August 2002, the following Gross Rent:

         Ten Thousand Dollars ($10,000.00) per month plus GST (as defined
         below).

         The Sublandlord covenants and agrees that, except as otherwise provided
         herein, the above noted Gross Rent is inclusive of any and all
         additional rent items (the "Additional Rent") noted and payable by the
         Sublandlord, including but not limited to all taxes, interior cleaning,
         heat, hydro, administration and general operating and maintenance costs
         as defined in the Head Lease.

         (a)  GOODS & SERVICES TAX: The Tenant shall be responsible for and pay
              directly to the Sublandlord all GOODS AND SERVICES TAXES incurred
              by Tenant for such goods and services -pursuant to Sublandlord's
              performance of its responsibilities under the Head Lease.

         (b)  INSURANCE. Sublandlord shall carry insurance as required and in
              accordance with the Head Lease in the amount of $5,000,000.

3.       TENANT INDEMNIFICATION:
         The Tenant covenants with the Sublandlord to indemnify and save
         harmless the Sublandlord from any and all claims made against the
         Sublandlord by the Landlord or others as a result of any renovations
         and/or physical adjustments made to the Leased Premises, any costs that
         may be incurred by the Sublandlord to restore the Leased Premises back
         to their original condition as may be required by the Landlord and any
         contravention by the Tenant of its covenants herein contained.

4.       SUBLANDLORD COVENANTS: The Sublandlord covenants and agrees with the
         Tenant:

         a)   to perform and observe the covenants on the Sublandlord's part
              under the Head Lease, so far as such covenants are not required to
              be performed and preserved by the Tenant, and at all times to keep
              the Tenant indemnified against all actions, expenses, claims and
              demands on account of the non-performance by the Sublandlord of
              its obligations.

         b)   to maintain the Head Lease in full force and effect during the
              Term and any renewal term of this Sublease subject, however, to
              any earlier termination of the Head Lease without the fault of
              Sublandlord

         c)   upon compliance with the terms and conditions of this Sublease,
              Tenant shall and may peaceably and quietly hold and enjoy the
              Leased Premises for the Term and any renewal term provided herein.
              The Sublandlord covenants and agrees with the Landlord that
              notwithstanding this Sublease the Sublandlord remains principally
              liable for performing all of the obligations on its part contained
              in the Head Lease.

5.       USE:
         The Leased Premises shall be used by the Tenant in accordance with the
         provisions of the Head Lease.

6.       CONDITION OF LEASED PREMISES:
         Upon the Commencement Date of this Sublease, Sublandlord shall tender
         the Leased Premises to the Tenant clean and in good condition and
         repair and shall remove all personal property and trade fixtures, other
         than any articles to remain by mutual agreement between Sublandlord and
         Tenant and detailed in Exhibit C.

         The Tenant agrees to accept the Leased Premises in the current "as is"
         condition.

                                       2
<PAGE>

         The Tenant shall, upon execution of this Sublease, pay to the
         Sublandlord the sum of $2,500.00 and for this sum the Sublandlord shall
         provide services and manpower to move furniture and to otherwise
         prepare the Leased Premises for the Tenant's occupancy.

7.       ALTERATIONS AND IMPROVEMENTS:
         Tenant shall not make any alterations or improvements to the Leased
         Premises of any kind without the prior written consent of the
         Sublandlord and Landlord as required by the Head Lease. Sublandlord
         warrants and certifies to Tenant that there are no pre-existing or
         otherwise outstanding issues regarding 'end of lease restorations'
         within the Building, which will accrue to Tenant, except as disclosed
         in the Head Lease. Should Tenant make any alterations that are required
         to be removed by the Landlord or the Sublandlord at the end of the
         Term, such removal shall be at the expense of the Tenant. If Tenant
         fails to perform the removal work after being called upon to do so by
         Landlord or Sublandlord, Landlord or Sublandlord may enter upon the
         Leased Premises and perform such removal work, and Tenant agrees to pay
         Landlord or Sublandlord the costs thereof.

8.       OBLIGATIONS OF TENANT:
         Tenant shall enjoy all rights granted to Sublandlord under the
         provisions of the Head Lease, except as expressly stated herein. Tenant
         covenants and agrees to abide by all the terms and conditions of this
         Sublease, and agrees for the benefit of the Sublandlord to abide by and
         perform all of the Head Lease terms and provisions except, to the
         extent they are different, as may be expressly provided by and agreed
         upon in this Sublease. Tenant covenants and agrees to pay the Gross
         Rent and moneys herein reserved, use the Leased Premises only for the
         purpose stated herein and only as provided under the Head Lease, and to
         surrender the Leased Premises upon the expiration or earlier
         termination of this Sublease Term, or any renewal term, in a condition
         equal to the condition of the Leased Premises as when received,
         approved alterations, reasonable wear and tear and casualty damage
         excepted.

9.       common areas:
         The Tenant shall have the use, in common with the Sublandlord and other
         tenants, if any, during the Term of this Sublease and any renewal term,
         of the following:

         Toilets and common hallways on the first floor.
         The reception area, defined as the entrance way, chairs and tables, but
         excluding the receptionist, reception desk and main switchboard and
         PBX.
         The lunchroom, including the kitchen, and shoe room.
         The shower facilities, excluding the training and fitness room.
         Any reference to the "common area" specifically excludes the training
         room and reading room adjacent, production / loading rooms mail room
         and other secure rooms on the first floor such as electrical room,
         elevator room, janitor closet, PBX / server room.
         The training room may be made available to the Tenant subject to the
         payment of current rental rates which shall be an additional charge.
         The second and third floors are not common areas.
         The cost for the use of the common areas is included in the Gross Rent.

10.      FURNITURE
         The Sublandlord agrees to provide the Tenant with furnishings to
         accommodate ten (10) offices and one meeting room as set out in Exhibit
         D. These furnishings will be made available to the Tenant upon the
         Commencement Date and will be used by the Tenant for the Term of this
         Sublease and any renewal term. All furniture and equipment is as shown
         in Exhibit D attached hereto and forming part of this Sublease.

                                       3
<PAGE>

         Upon expiry of the Sublease Term and/or any renewal term, the Tenant
         shall return the furnishings to the Sublandlord in the condition it was
         in when received by the Tenant, normal wear and tear excepted.

11.      HOLD HARMLESS: Tenant, unless due to the negligence or willful
         misconduct of Sublandlord [and those for whom Sublandlord is
         responsible in law], agrees to indemnify, defend, and hold Sublandlord
         harmless from and against any and all liability, penalties, losses,
         damages, costs and expenses, demands, causes of action, claims or
         judgments arising from or growing out of any injury to any person or
         persons or any damage to any property in the Leased Premises and from
         and against all legal cost and charges, including reasonable attorneys'
         fees incurred with respect to any or such matters.

         Unless due to Tenant's negligence or willful misconduct, Sublandlord
         agrees to indemnify, defend and hold Tenant harmless from and against
         any and all liability, penalties, losses, damages, costs and expenses,
         demands, causes of action, claims or judgments arising from or growing
         out of any injury to any person or persons or any damage to any
         property in Sublandlord's Leased Premises and from and against all
         legal costs and charges, including reasonable attorneys' fees incurred
         with respect to any or such matters.

12.      LAWS OF ONTARIO:
         This Sublease shall be deemed to have been made and shall be construed
         in accordance with the laws of the Province of Ontario.

13.      NO REPRESENTATIONS:
         The Sublandlord and Tenant agree that there are no covenants,
         representations, agreements, warranties or conditions in any way
         relating to the subject matter of this Sublease whether express or
         implied, collateral or otherwise, except those set forth in this
         Sublease.

14.      RENEWAL:
         The Tenant may elect one of the following two renewal options. There
         shall be no further right to renew this Sublease.

         Option One:
         At the end of the initial 13 month Term, unless 90 days advance written
         notice has been given to the Sublandlord of the Tenant's intention to
         not renew this Sublease, the Sublease will continue on a month to month
         basis at a Gross Rental amount equal to Twelve Thousand Dollars
         ($12,000.00) per month, plus GST, with all other terms and conditions
         of the Sublease remaining unchanged, until the Sublease is terminated
         by either party by providing to the other party 90 days prior written
         notice of such termination.

         Option Two:
         Tenant shall have the option to renew this Sublease for an additional
         term of one (1) year at a Gross Rental amount equal to Ten Thousand
         Eight Hundred Dollars ($10,800.00) per month plus GST. During the one
         (1) year renewal term, all terms and conditions of the Sublease shall
         remain unchanged. The Sublease shall terminate at the conclusion of the
         one (1) year renewal term. If Option Two is selected, the Tenant shall
         provide to the Sublandlord written notice at least 90 days prior to the
         expiry of the initial 13 month Term that it wishes to renew this
         Sublease for the additional one (1) year term. If the 90 day advance
         notice is not received, this renewal option shall expire.

         Other than with respect to the two (2) renewal options set out herein,
         the Landlord's consent and agreement is required for any further
         renewals of this Sublease or for any additional subletting of the Head
         Lease.

15.      PARKING:
         During the Term of the Sublease and any renewal term, the Tenant shall
         have access to 20% of

                                       4
<PAGE>

         the on-site parking (20% of each parking section). Parking for the Term
         of the Sublease and any renewal term is to be free of charge to the
         Tenant.

16.      SIGNAGE:
         In the event an exterior pylon sign is constructed, the Tenant shall be
         permitted to install its corporate identification signage on the pylon.
         The location and size shall be subject to Landlord and the
         Sublandlord's approval in accordance with the Head Lease. The
         installation and future removal and repairs, if any, of said signage
         shall be at the Tenant's expense. The signage shall not exceed 20% of
         the total signage allowed.

17.      ENTIRE AGREEMENT:
         The Sublandlord and the Tenant agree that this Sublease along with any
         Exhibits attached hereto constitutes the entire agreement between the
         Sublandlord ,and the Tenant. This agreement supersedes all oral
         representations made by the agent, Landlord and Sublandlord. This
         Sublease shall be deemed to contain all the provisions of the Head
         Lease, except those that are inconsistent with the provisions of this
         Sublease, as though they were set forth and contained in this Sublease.
         All references to "Landlord" and "Tenant" therein shall read
         "Sublandlord" and "Tenant" for the purposes of the relationship between
         the Sublandlord and the Tenant.

18.      TENANT DEFAULT:
         Notwithstanding anything to the contrary in the Head Lease, if; (i)
         Tenant fails to pay any rent, (gross or otherwise) when due, and should
         such failure continue for more than five (5) business days after
         receipt by Tenant of written notice thereof from Sublandlord, (however,
         in no event shall Sublandlord be required to issue such late payment
         notice more than once in any 12 month period), or (ii) Tenant defaults
         in the prompt and full performance of any provision of this Sublease,
         and should Tenant have failed to cure said performance within thirty
         (30) days or promptly fail to begin such compliance within thirty (30)
         days after receipt of written notice thereof from Sublandlord,
         Sublandlord may, at its option, terminate the Sublease by providing
         Tenant ten (10) business days notice in writing and Sublandlord may
         immediately retake possession of the Leased Premises with no recourse
         available by Tenant. In the event Sublandlord exercises its right to
         retake the Leased Premises, Tenant shall not be relieved of any of its
         obligations, including financial obligations, under this Sublease.

19.      PERMITS:
         Tenant shall be responsible for obtaining any and all required permits
         related to Tenants possession or alteration of the Leased Premises.

20.      NOTICE:
         All notices or other communications required or desired to be given
         hereunder shall be in writing and given either upon (a) personal
         delivery or (b) by nationally recognized overnight courier, or
         certified mailed postage prepaid, registered or certified mail, return
         receipt requested, or (c) by prepaid telegram, confirmed facsimile
         transmission or confirmed electronic mail followed by a confirmatory
         letter, addressed to the parties at the addresses set forth herein.
         Notices deposited in the mail or otherwise given in the manner
         described above shall be effective upon receipt. If to Sublandlord: 7
         Capella Court, Ottawa, Ontario, K2E 7X1 Telephone; 613-224-4700,
         Facsimile: 613-224-4706, chris.boland@optiwave.com Attn: Chief
         Financial Officer. If to Tenant: 7 Capella Court, Nepean, Ontario,
         Attn: _________ with a copy to Zixit Corporation, Attn: Chief Financial
         Officer, 2711 North Haskell Avenue, Suite 2300, LB 36, Dallas, Texas
         75204-2960 U.S.A, (214) 370-2000 telephone, (214) 515-7385 facsimile,
         syork@zixit.com. Any party may change its address for - notice by
         giving notice in the manner hereinabove provided.

21.      WAIVER:
         The waiver of any breach of any term, covenant or condition herein
         contained shall not be deemed to be a waiver of any other term,
         covenant or condition contained herein.

                                       5
<PAGE>

22.      COMPLIANCE WITH LEASE:
         Sublandlord represents and warrants to Tenant that (a) Sublandlord has
         delivered to Tenant a full and complete copy of the Head Lease and all
         other agreements between Landlord and Sublandlord relating to the
         leasing, use and occupancy of the Leased Premises; (b) the Head Lease
         is in full force and effect and (c) to the best of its knowledge, there
         are no defaults on Sublandlord's part under the Head Lease as of the
         Commencement Date of this Sublease and no event has occurred and is
         continuing which would constitute an event of default [but for the
         requirement of the giving of notice and/or the expiration of the period
         of time to cure]. Sublandlord and Landlord represent and warrant that
         there are no outstanding matters, issues, claims, disputes of any
         nature whatsoever between them in connection with the Head Lease.

23.      ACCESS:
         The Tenant shall have access to the Leased Premises and the common
         areas defined in section 9, twenty-four hours per day, seven days per
         week.

24.      CASUALTY DAMAGE:
         If the Property, Building, demises premises or Leased Premises shall be
         damaged or destroyed by fire or other casualty or if all or party of
         the Property, Building, demises premises or Leased Premises shall be
         taken over by eminent domain to such an extent as to render the Leased
         Premises untenantable in whole or in part, the Gross Rent shall be
         abated in the same proportion that the portion of the Leased Premises
         rendered unfit for occupancy by Tenant shall bear to the whole of the
         Leased Premises. If the damage or taking reasonably precludes occupancy
         of any substantial part of the Leased Premises and if the estimated
         repair time to restore that part of the Leased Premises to a condition
         that reasonably permits occupancy will extend beyond 120 days after the
         date of the casualty or taking, Tenant may elect to terminate this
         Sublease by so notifying Sublandlord within 10 days after receipt from
         Sublandlord of Landlord's decision not to terminate the Lease. If the
         Lease shall be terminated as a consequence of such damage, destruction
         or taking, this Sublease shall terminate.

25.      ATTORNEY'S FEES:
         In the event that any action shall be commenced by either party hereto
         arising out of or concerning this Sublease or any right or obligation
         derived therefrom, then in addition to all other relief at law or in
         equity the prevailing party shall be entitled to recover reasonable
         attorney's fees and costs as fixed by the court.

26.      ENUREMENT:
         This Sublease shall inure to the benefit of and be binding upon the
         Sublandlord, the Tenant and their respective heirs, executors,
         administrators, successors and permitted assigns.

27.      FACSIMILE TRANSMISSION:
         Acceptance of this Sublease may be communicated by facsimile
         transmission or by delivery of such facsimile without limiting other
         methods of communicating acceptance available to the parties.

28.      early access period:
         The Tenant shall have early access to the Leased Premises upon the
         waiving of the condition to this Sublease as noted in Section 30 herein
         for the purpose of preparing the Leased Premises for their business.
         This early access period shall be completely Gross Rent Free to the
         Tenant. the early access period shall begin July 29, 2002.

29.      SUBORDINATION:
         This Sublease shall be subject and subordinate at all times to the
         mortgages registered against the Building and to the Head Lease and all
         of its provisions, covenants and conditions. The Tenant and Sublandlord
         shall not commit or fail to do any act, which would cause or constitute
         an event or default under the Head Lease.

                                       6
<PAGE>

30.      CONDITION:
         This Sublease is conditional upon the Sublandlord obtaining from the
         Landlord, approval as to the terms and conditions herein and in the
         headlease. By executing this Agreement in the space provided below, the
         Landlord shall have confirmed consent to and approve the Sublease and
         headlease.

         If the Sublandlord fails to notify the Tenant in writing that this
         condition has been satisfied or waived within ten (10) business days of
         acceptance of this Sublease, then the agreement contemplated herein
         will become null and void and neither party shall have further
         obligation to the other.

31.      DEPOSIT:
         The Tenant shall provide a Deposit cheque equal to three (3) months
         Gross Rent plus GST (the "Deposit") payable to Colliers Macaulay
         Nicolls (Ontario) Inc. as Agent ("Agent"), within forty-eight (48)
         hours of acceptance of this Sublease. Such Deposit will be held in
         trust for both parties until all conditions to this Sublease are either
         waived or satisfied, whereupon the parties hereby authorize and direct
         the Agent to release the Deposit from trust and apply the Deposit to
         the commissions payable to the Agent pursuant to Section 32 below, and
         to pay the balance of the Deposit, if any, to the Landlord, to be
         applied to the Sublandlord's rental account. The Agent shall advise the
         Sublandlord in writing and provide details of such application and the
         Landlord shall immediately apply the amount of the Deposit [, or
         balance thereof,] to the payment of the Gross Rent for the months of
         September and October 2002 and August 2003.

         The Agent shall immediately return the Deposit to the Tenant without
         interest, deduction or set-off if any of the conditions are neither
         waived nor satisfied.

32.      PROFESSIONAL FEES:
         The Tenant warrants that it has had no dealings with any real estate
         broker or agent in connection with the negotiation of this Sublease
         other than the Agent, and that it knows of no other real estate broker
         or agent who is or might be entitled to a commission in connection with
         this Sublease other than the Agent, whose fees shall be paid by the
         Sublandlord upon the date set for occupancy by the Tenant. For greater
         certainty, the Landlord shall have no obligation to make any payment to
         the Agent on account of any real estate commission.

         The Agent has acted on both the selling side and the listing side of
         this transaction. The fees due to the Agent shall be due and payable
         upon the date of occupancy, of August l, 2002.

         IN WITNESS WHEREOF, the parties have executed this Sublease, under
         seal, as of the date written above.

         I the undersigned Tenant agree to and accept the above Sublease.

SIGNED, SEALED AND DELIVERED      ) ZIXIT CORPORATION
in the presence of:               )
                                  )
                                  )
/s/ Anne Paliwal                  ) Per: /s/ Wael Mohamed
------------------------------    )      -------------------------------
Witness                           ) Title:  VP of Global Services
                                  ) I/We have the authority to bind
                                    the corporation.

                                       7
<PAGE>

I the undersigned Sublandlord agree to and accept the above Sublease.

SIGNED, SEALED AND DELIVERED      ) OPTIWAVE CORPORATION/CORPORATION OPTIWAVE
In the presence of:               )
                                  )
                                  )
/s/ Curtis Flanagan               ) Per:  /s/ Chris Boland
-----------------------------     )     -----------------------------------
Witness                           ) Title: VP Finance
                                  ) I/We have the authority to bind
                                    the corporation.

I the undersigned Landlord agree to and accept the above Sublease.

SIGNED, SEALED AND DELIVERED      ) WAIDT CONSTRUCTION & DEVELOPMENTS LTD.
in the presence of:               )
                                  )
                                  )
                                  ) Per: /s/ Trevor Woodside
-----------------------------     )     -----------------------------------
Witness                           ) Title:
                                  ) I/We have the authority to bind the
                                    corporation.

                                       8

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