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                                                                   EXHIBIT 10.25

         THIS SUBLEASE AMENDMENT AGREEMENT made the 19th day of November 2002
BETWEEN:

                    OPTIWAVE CORPORATION/CORPORATION OPTIWAVE

         (a Canadian corporation, hereinafter called the "Sublandlord") OF THE
         FIRST PART
AND:

                                 ZIX CORPORATION
         (a Texas, U.S.A. corporation, hereinafter called the "Tenant") OF THE
         SECOND PART


AND                  WAIDT CONSTRUCTION & DEVELOPMENTS LTD

         (hereinafter called the "Landlord") OF THE THIRD PART

         WHEREAS by written Sublease dated August 1, 2002 and by a Sublease
Amendment Agreement dated September 30, 2002 between the Sublandlord, the Tenant
and the Landlord, the Tenant subleased space on the ground floor of a building
located at 7 Capella Court, Nepean, Ontario ("the Building").

         AND WHEREAS the Tenant and the Sublandlord have agreed to further amend
the Sublease so as to sublet to the Tenant, effective December 16, 2002, the
entire third floor of the Building as shown on the floor plan attached hereto
and marked as Exhibit A and incorporated herein by reference (the "Third Floor
Leased Premises"), which Third Floor Leased Premises is intended to replace the
existing Leased Premises and Additional Leased Premises located on the ground
floor of the Building, which the Tenant agrees to vacate by December 15, 2002.

         AND WHEREAS the Landlord has agreed to amend the Sublease to replace
and substitute the ground floor Leased Premises and Additional Leased Premises
with the Third Floor 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.       Except for the amendments expressly set out herein, the Parties confirm
         that all other terms and conditions contained in the Sublease remain in
         full force and effect. More specifically, if the Sublease is not
         renewed pursuant to its terms, the Tenant shall vacate the Third Floor
         Leased Premises by August 31, 2003.

2.       The Gross Rent payable by the Tenant pursuant to the Sublease Agreement
         dated August 1, 2002 and the Sublease Amendment Agreement dated
         September 30, 2002 shall remain unchanged for November 2002. On the
         express consent of the parties hereto, the Sublease Amendment Agreement
         dated September 30, 2002 shall terminate on December 15, 2002.
         Thereafter, the Gross Rent payable by the Tenant specified in the
         Sublease will be increased by $15,000.00 per month, plus GST, (to
         $25,000.00 per month plus GST) as payment for the Tenant's possession
         and occupation of the Third Floor Leased Premises identified on the
         floor plan in Exhibit A. (Except for December month, which shall be
         prorated for half of the month, increasing by $8,750 plus GST to
         $18,750 plus GST). If the Sublease is renewed on the basis of Option
         One, the monthly Gross Rent payable shall be $30,000.00 for the
         duration of any such renewal term. If the Sublease is renewed on the
         basis of Option Two, the monthly Gross Rent payable shall be $27,000.00
         for the duration of the one (1) year renewal term. Terms of payment

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         shall remain the same as specified in the Sublease with the first
         increased payment on account of the Third Floor Leased Premises due on
         December 16, 2002.

         On December 16, 2002 the Tenant shall, in addition, advance to the
         Landlord the sum of $12,500.00 to be applied to "top up" to $25,000.00
         the Tenant's final months payment of Gross Rent for August 2003.

3.       The Tenant shall vacate the ground floor Leased Premises and Additional
         Leased Premises no later than December 15, 2002 and thereafter the
         Tenant shall have no leasehold interest of any description in or to the
         ground floor of the Building. The Tenant may enter into possession of
         the Third Floor Leased Premises prior to December 16, 2002 provided
         this Sublease Amendment Agreement is fully executed.

4.       The Third Floor Leased Premises are sublet to the Tenant in an "as is"
         condition with no requirement for any additional fit up, alterations,
         provision of furniture or equipment, use of maintenance resources or
         leasehold improvements. There are no inducements offered to the Tenant
         and no free services will be provided in connection with the Tenant's
         occupation of the third floor of the Building. More specifically, the
         Tenant will not receive the benefit of any phone system in connection
         with its occupation of the Third Floor Leased Premises and the Tenant
         will be required to purchase or lease, at their sole expense, a phone
         system that meets their requirements.

5.       The common areas defined in the Sublease shall be amended to include
         the toilets and common hallways on the third floor. The second floor is
         not a common area.

6.       GROSS RENT: For greater certainty, Tenant shall pay directly to the
         Landlord all gross rents, prepayments and deposits to be applied to the
         Sublandlord's rental account, as Gross Rent for the Third Floor Leased
         Premises, payable in equal monthly installments in advance on the first
         day of each calendar month during the term of the Sublease.

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

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

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


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

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

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

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

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