<PAGE>
As filed with the Securities and Exchange Commission on April 15, 2003.
Registration No. 333-__________.
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                    FORM S-8

                        REGISTRATION STATEMENT UNDER THE
                             SECURITIES ACT OF 1933

                                 ZIX CORPORATION
             (Exact Name of Registrant as Specified in Its Charter)

               Texas                                     75-2216818
   (State or other jurisdiction                      (I.R.S. Employer
  of incorporation or organization)                 Identification No.)

                                                        Steve M. York
2711 N. Haskell Avenue                              2711 N. Haskell Avenue
     Suite 2850, LB 36                                 Suite 2850, LB 36
Dallas, Texas 75204-2911                            Dallas, Texas 75204-2911

                                                        (214) 515-7300
(Address, including zip code,                          (Name, address,
including zip code,                                  and telephone number,
of principal executive offices)                   code, of agent for service)
including area

                ZIX CORPORATION 1999 DIRECTORS' STOCK OPTION PLAN

                            (FULL TITLE OF THE PLAN)

                                   ----------

                         CALCULATION OF REGISTRATION FEE

<Table>
<Caption>
        TITLE OF                 AMOUNT             PROPOSED MAXIMUM       PROPOSED MAXIMUM           AMOUNT OF
       SECURITIES                 TO BE              OFFERING PRICE            AGGREGATE            REGISTRATION
    TO BE REGISTERED          REGISTERED(1)           PER SHARE(2)         OFFERING PRICE(2)           FEE(2)
------------------------- ---------------------- ----------------------- ---------------------- ----------------------
<S>                       <C>                    <C>                     <C>                    <C>
 Common Stock,                 225,000 Shares              $4.39                 $987,750                $79.91
 $.01 par value
</Table>

(1)        Securities being registered consist of 225,000 shares issuable upon
           exercise of options under the Zix Corporation 1999 Directors' Stock
           Option Plan (the "Directors' Plan"). Pursuant to Rule 416(c) under
           the Securities Act of 1933, as amended (the "Securities Act"), this
           Registration Statement also covers any additional shares of Common
           Stock that may be issuable pursuant to the anti-dilution provisions
           of the Directors' Plan.

(2)        Estimated solely for the purpose of calculating the registration fee.
           The registration fee has been calculated in accordance with Rules
           457(c) and 457(h) under the Securities Act on the basis of the
           average of the high and low prices of the Common Stock as quoted on
           the NASDAQ National Market System on April 10, 2003, which was $4.39.



<PAGE>



                              EXPLANATORY STATEMENT

         Shares of Common Stock of the Registrant for issuance upon exercise of
options under the Directors' Plan have heretofore been registered under a
Registration Statement on Form S-8 No. 333-90323. This Registration Statement on
Form S-8 registers an additional 225,000 shares of our common stock available
for issuance under the Directors' Plan.

                                     PART I

         The information specified in Part I of Form S-8 is contained in
documents sent or given to option holders in accordance with Rule 428 under the
Securities Act. Such documents and the documents incorporated by reference
pursuant to Item 3 of Part II of this Registration Statement, taken together,
constitute the Section 10(a) prospectus.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE.

         The following documents are hereby incorporated herein by reference:

         (a) The Registrant's Annual Report on Form 10-K, including audited
financial statements, for the Registrant's fiscal year ended December 31, 2002;

         (b) All other reports filed by the Registrant pursuant to Section 13(a)
or 15(d) of the Securities Exchange Act of 1934 (the "Exchange Act") since the
end of the fiscal year covered by the Annual Report referred to in (a) above;
and

         (c) A description of the Registrant's Common Stock contained in the
Registrant's Registration Statement on Form 8-A, filed with the Securities and
Exchange Commission on September 25, 1989, including any amendment or report
filed for the purpose of updating such description.

         All reports or other documents subsequently filed by the Registrant
pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to
the filing of a post-effective amendment which indicates that all securities
offered have been sold or which deregisters all securities then remaining
unsold, shall be deemed to be incorporated by reference in this Registration
Statement and to automatically update and supersede the information contained in
this Registration Statement.

         Any statement contained herein or in a document incorporated or deemed
to be incorporated herein by reference shall be deemed to be modified or
superseded for purposes of this Registration Statement to the extent that a
statement contained herein or in any other subsequently filed document which
also is or is deemed to be incorporated herein by reference modifies or
supersedes such earlier statement. Any statement so modified or superseded shall
not be deemed, except as so modified or superseded, to constitute a part of this
Registration Statement.



                                      II-2
<PAGE>



ITEM 4. DESCRIPTION OF SECURITIES.

         Not applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

         Not applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         As permitted by the Texas Business Corporation Act, the Registrant's
Articles of Incorporation provide that directors of the Registrant shall not be
personally liable to the Registrant or its shareholders for monetary damages for
breach of fiduciary duty as a director, except for liability for (i) any breach
of the director's duty of loyalty to the Registrant or its shareholders, (ii)
any act or omission not in good faith or which involves intentional misconduct
or a knowing violation of law, (iii) any transaction from which the director
derived any improper personal benefit, (iv) any act or omission where the
liability of the director is expressly provided by statute, or (v) any act
related to an unlawful stock repurchase or payment of a dividend. In addition,
the Registrant's Articles of Incorporation and Bylaws include certain provisions
permitted by the Texas Business Corporation Act whereby directors, officers,
employees, and agents of the Registrant generally are to be indemnified against
certain liabilities to the fullest extent authorized by the Texas Business
Corporation Act.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

ITEM 8. EXHIBITS.

         The Exhibits to this Registration Statement are listed in the Index to
Exhibits on page II-7 of this Registration Statement, which Index is
incorporated herein by reference.

ITEM 9. UNDERTAKINGS.

         (a) The undersigned Registrant hereby undertakes:

                  (1) To file, during any period in which offers or sales are
         being made, a post-effective amendment to this Registration Statement:

                           (i) To include any Prospectus required by Section
                  10(a)(3) of the Securities Act;

                           (ii) To reflect in the Prospectus any facts or events
                  arising after the effective date of the Registration Statement
                  (or the most recent post-effective amendment thereof) which,
                  individually or in the aggregate, represent a fundamental
                  change in the information set forth in the Registration
                  Statement;

                           (iii) To include any material information with
                  respect to the plan of distribution not previously disclosed
                  in the Registration Statement or any material change to such
                  information in the Registration Statement;



                                      II-3
<PAGE>



         provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) shall not
         apply if the information required to be included in a post-effective
         amendment by those paragraphs is contained in periodic reports filed by
         the Registrant pursuant to Section 13 or Section 15(d) of the Exchange
         Act that are incorporated by reference in the Registration Statement.

                  (2) That, for the purpose of determining any liability under
         the Securities Act, each such post-effective amendment shall be deemed
         to be a new registration statement relating to the securities offered
         therein, and the offering of such securities at that time shall be
         deemed to be the initial bona fide offering thereof.

                  (3) To remove from registration by means of a post-effective
         amendment any of the securities being registered which remain unsold at
         the termination of the offering.

         (b) The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's Annual Report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act) that is
incorporated by reference in the Registration Statement shall be deemed to be a
new registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

         (c) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Registrant pursuant to the provisions described in Item 6 or otherwise,
the Registrant has been advised that in the opinion of the Securities and
Exchange Commission, such indemnification is against public policy as expressed
in the Securities Act and is, therefore, unenforceable. In the event that a
claim for indemnification against such liabilities (other than the payment by
the Registrant of expenses incurred or paid by a director, officer or
controlling person of the Registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the Registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act and will be governed by the final adjudication of such issue.



                                      II-4
<PAGE>



                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Dallas, State of Texas on April 14, 2003.


                                     ZIX CORPORATION


                                     By: /s/ Steve M. York
                                         --------------------------------------
                                         Steve M. York
                                         Senior Vice President, Chief Financial
                                         Officer and Treasurer

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities indicated on April 14, 2003.

<Table>
<Caption>
               SIGNATURE                             TITLE
               ---------                             -----
<S>                                         <C>
/*/                                         President, Chief
------------------------------------        Executive Officer and
(John A. Ryan)                              Director (Principal Executive
                                            Officer)

/s/ Steve M. York                           Senior Vice President,
------------------------------------        Chief Financial Officer
(Steve M. York)                             and Treasurer (Principal
                                            Financial and Accounting
                                            Officer)

/*/                                         Director
------------------------------------
(Michael E. Keane)

/*/                                         Director
------------------------------------
(James S. Marston)

/*/                                         Director
------------------------------------
(Antonio R. Sanchez, Jr.)

/*/                                         Director
------------------------------------
(Dr. Ben G. Streetman)
</Table>


*By:     /s/ Steve M. York
         ------------------
         Steve M. York
         Attorney-in-Fact



                                      II-5
<PAGE>



                                INDEX TO EXHIBITS

<Table>
<Caption>
Exhibit
Number            Description
------            -----------
<S>               <C>
4.1*              1999 Directors' Stock Option Plan of the Registrant, effective
                  as of August 1, 2002.

5.1*              Opinion of Ronald A. Woessner as to the validity of the
                  securities being registered.

23.1              Consent of Ronald A. Woessner (included in his opinion filed
                  as Exhibit 5.1).

23.2*             Consent of Ernst & Young LLP.

24.1              Power of Attorney (included in Part II of this Registration
                  Statement).
</Table>

----------
*Filed electronically herewith.



                                      II-6
[{"type": "S-8", "sequence": "1", "filename": "d04894sv8.txt", "description": "FORM S-8", "secsgml_size_bytes": 14798}]
{"company-data": {"conformed-name": "ZIX CORP", "cik": "0000855612", "assigned-sic": "7370", "irs-number": "752216818", "state-of-incorporation": "TX", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-104532", "film-number": "03649329"}, "business-address": {"street1": "2711 NORTH HASKELL AVENUE", "street2": "SUITE 2300, LB 36", "city": "DALLAS", "state": "TX", "zip": "75204-2960", "phone": "214-370-2000"}, "mail-address": {"street1": "2711 NORTH HASKELL AVENUE", "street2": "SUITE 2300, LB 36", "city": "DALLAS", "state": "TX", "zip": "75204-2960"}, "former-company": [{"former-conformed-name": "AMTECH CORP", "date-changed": "19920703"}, {"former-conformed-name": "CUSTOMTRACKS CORP /TX/", "date-changed": "19980904"}, {"former-conformed-name": "ZIXIT CORP", "date-changed": "19991013"}]}
As filed with the Securities and Exchange Commission on April 15, 2003.
Registration No. 333-__________.
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                    FORM S-8

                        REGISTRATION STATEMENT UNDER THE
                             SECURITIES ACT OF 1933

                                 ZIX CORPORATION
             (Exact Name of Registrant as Specified in Its Charter)

               Texas                                     75-2216818
   (State or other jurisdiction                      (I.R.S. Employer
  of incorporation or organization)                 Identification No.)

                                                        Steve M. York
2711 N. Haskell Avenue                              2711 N. Haskell Avenue
     Suite 2850, LB 36                                 Suite 2850, LB 36
Dallas, Texas 75204-2911                            Dallas, Texas 75204-2911

                                                        (214) 515-7300
(Address, including zip code,                          (Name, address,
including zip code,                                  and telephone number,
of principal executive offices)                   code, of agent for service)
including area

                ZIX CORPORATION 1999 DIRECTORS' STOCK OPTION PLAN

                            (FULL TITLE OF THE PLAN)

                                   ----------

                         CALCULATION OF REGISTRATION FEE
TITLE OF                 AMOUNT             PROPOSED MAXIMUM       PROPOSED MAXIMUM           AMOUNT OF
       SECURITIES                 TO BE              OFFERING PRICE            AGGREGATE            REGISTRATION
    TO BE REGISTERED          REGISTERED(1)           PER SHARE(2)         OFFERING PRICE(2)           FEE(2)
------------------------- ---------------------- ----------------------- ---------------------- ----------------------
Common Stock,                 225,000 Shares              $4.39                 $987,750                $79.91
 $.01 par value
(1)        Securities being registered consist of 225,000 shares issuable upon
           exercise of options under the Zix Corporation 1999 Directors' Stock
           Option Plan (the "Directors' Plan"). Pursuant to Rule 416(c) under
           the Securities Act of 1933, as amended (the "Securities Act"), this
           Registration Statement also covers any additional shares of Common
           Stock that may be issuable pursuant to the anti-dilution provisions
           of the Directors' Plan.

(2)        Estimated solely for the purpose of calculating the registration fee.
           The registration fee has been calculated in accordance with Rules
           457(c) and 457(h) under the Securities Act on the basis of the
           average of the high and low prices of the Common Stock as quoted on
           the NASDAQ National Market System on April 10, 2003, which was $4.39.
EXPLANATORY STATEMENT

         Shares of Common Stock of the Registrant for issuance upon exercise of
options under the Directors' Plan have heretofore been registered under a
Registration Statement on Form S-8 No. 333-90323. This Registration Statement on
Form S-8 registers an additional 225,000 shares of our common stock available
for issuance under the Directors' Plan.

                                     PART I

         The information specified in Part I of Form S-8 is contained in
documents sent or given to option holders in accordance with Rule 428 under the
Securities Act. Such documents and the documents incorporated by reference
pursuant to Item 3 of Part II of this Registration Statement, taken together,
constitute the Section 10(a) prospectus.

                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE.

         The following documents are hereby incorporated herein by reference:

         (a) The Registrant's Annual Report on Form 10-K, including audited
financial statements, for the Registrant's fiscal year ended December 31, 2002;

         (b) All other reports filed by the Registrant pursuant to Section 13(a)
or 15(d) of the Securities Exchange Act of 1934 (the "Exchange Act") since the
end of the fiscal year covered by the Annual Report referred to in (a) above;
and

         (c) A description of the Registrant's Common Stock contained in the
Registrant's Registration Statement on Form 8-A, filed with the Securities and
Exchange Commission on September 25, 1989, including any amendment or report
filed for the purpose of updating such description.

         All reports or other documents subsequently filed by the Registrant
pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to
the filing of a post-effective amendment which indicates that all securities
offered have been sold or which deregisters all securities then remaining
unsold, shall be deemed to be incorporated by reference in this Registration
Statement and to automatically update and supersede the information contained in
this Registration Statement.

         Any statement contained herein or in a document incorporated or deemed
to be incorporated herein by reference shall be deemed to be modified or
superseded for purposes of this Registration Statement to the extent that a
statement contained herein or in any other subsequently filed document which
also is or is deemed to be incorporated herein by reference modifies or
supersedes such earlier statement. Any statement so modified or superseded shall
not be deemed, except as so modified or superseded, to constitute a part of this
Registration Statement.

                                      II-2
ITEM 4. DESCRIPTION OF SECURITIES.

         Not applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

         Not applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         As permitted by the Texas Business Corporation Act, the Registrant's
Articles of Incorporation provide that directors of the Registrant shall not be
personally liable to the Registrant or its shareholders for monetary damages for
breach of fiduciary duty as a director, except for liability for (i) any breach
of the director's duty of loyalty to the Registrant or its shareholders, (ii)
any act or omission not in good faith or which involves intentional misconduct
or a knowing violation of law, (iii) any transaction from which the director
derived any improper personal benefit, (iv) any act or omission where the
liability of the director is expressly provided by statute, or (v) any act
related to an unlawful stock repurchase or payment of a dividend. In addition,
the Registrant's Articles of Incorporation and Bylaws include certain provisions
permitted by the Texas Business Corporation Act whereby directors, officers,
employees, and agents of the Registrant generally are to be indemnified against
certain liabilities to the fullest extent authorized by the Texas Business
Corporation Act.

ITEM 7.  EXEMPTION FROM REGISTRATION CLAIMED.

         Not applicable.

ITEM 8. EXHIBITS.

         The Exhibits to this Registration Statement are listed in the Index to
Exhibits on page II-7 of this Registration Statement, which Index is
incorporated herein by reference.

ITEM 9. UNDERTAKINGS.

         (a) The undersigned Registrant hereby undertakes:

                  (1) To file, during any period in which offers or sales are
         being made, a post-effective amendment to this Registration Statement:

                           (i) To include any Prospectus required by Section
                  10(a)(3) of the Securities Act;

                           (ii) To reflect in the Prospectus any facts or events
                  arising after the effective date of the Registration Statement
                  (or the most recent post-effective amendment thereof) which,
                  individually or in the aggregate, represent a fundamental
                  change in the information set forth in the Registration
                  Statement;

                           (iii) To include any material information with
                  respect to the plan of distribution not previously disclosed
                  in the Registration Statement or any material change to such
                  information in the Registration Statement;

                                      II-3
provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) shall not
         apply if the information required to be included in a post-effective
         amendment by those paragraphs is contained in periodic reports filed by
         the Registrant pursuant to Section 13 or Section 15(d) of the Exchange
         Act that are incorporated by reference in the Registration Statement.

                  (2) That, for the purpose of determining any liability under
         the Securities Act, each such post-effective amendment shall be deemed
         to be a new registration statement relating to the securities offered
         therein, and the offering of such securities at that time shall be
         deemed to be the initial bona fide offering thereof.

                  (3) To remove from registration by means of a post-effective
         amendment any of the securities being registered which remain unsold at
         the termination of the offering.

         (b) The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's Annual Report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act) that is
incorporated by reference in the Registration Statement shall be deemed to be a
new registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

         (c) Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Registrant pursuant to the provisions described in Item 6 or otherwise,
the Registrant has been advised that in the opinion of the Securities and
Exchange Commission, such indemnification is against public policy as expressed
in the Securities Act and is, therefore, unenforceable. In the event that a
claim for indemnification against such liabilities (other than the payment by
the Registrant of expenses incurred or paid by a director, officer or
controlling person of the Registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the Registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act and will be governed by the final adjudication of such issue.

                                      II-4
SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Dallas, State of Texas on April 14, 2003.

                                     ZIX CORPORATION

                                     By: /s/ Steve M. York
                                         --------------------------------------
                                         Steve M. York
                                         Senior Vice President, Chief Financial
                                         Officer and Treasurer

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities indicated on April 14, 2003.
SIGNATURE                             TITLE
               ---------                             -----
/*/                                         President, Chief
------------------------------------        Executive Officer and
(John A. Ryan)                              Director (Principal Executive
                                            Officer)

/s/ Steve M. York                           Senior Vice President,
------------------------------------        Chief Financial Officer
(Steve M. York)                             and Treasurer (Principal
                                            Financial and Accounting
                                            Officer)

/*/                                         Director
------------------------------------
(Michael E. Keane)

/*/                                         Director
------------------------------------
(James S. Marston)

/*/                                         Director
------------------------------------
(Antonio R. Sanchez, Jr.)

/*/                                         Director
------------------------------------
(Dr. Ben G. Streetman)
*By:     /s/ Steve M. York
         ------------------
         Steve M. York
         Attorney-in-Fact

                                      II-5
INDEX TO EXHIBITS
Exhibit
Number            Description
------            -----------
4.1*              1999 Directors' Stock Option Plan of the Registrant, effective
                  as of August 1, 2002.

5.1*              Opinion of Ronald A. Woessner as to the validity of the
                  securities being registered.

23.1              Consent of Ronald A. Woessner (included in his opinion filed
                  as Exhibit 5.1).

23.2*             Consent of Ernst & Young LLP.

24.1              Power of Attorney (included in Part II of this Registration
                  Statement).
----------
*Filed electronically herewith.

                                      II-6