Exhibit 10.37

September 12, 2004

Zix Corporation
2711 N. Haskell Avenue
Suite 2300, LB 36
Dallas, Texas 75204-2960
Attention: Legal Department

RE: Master Services Agreement Between Aventis Inc. and Zix Corporation dated
January 30, 2004 (“Agreement”)

Dear Ron:

               As discussed, and pursuant to section 1.1 of Article 1, the
following services shall be added to Schedule B of the above referenced
Agreement and available for Aventis, at Aventis’ sole discretion, to procure
pursuant to the Agreement.

Secure email

ZixVPM — ZixVPM (Virtual Private Messenger) is a service that enables
organizations to implement a company wide system for sending and receiving
encrypted email.

ZixPort — This fully integrated service provides your existing portal with the
functionality for sending and receiving encrypted email.

ZixMail — A desktop application that enables individuals to send and receive
encrypted email to anyone quickly and easily.

Assessment Services

ZixAuditor — An assessment service that identifies, quantifies, and reports
email vulnerabilities and monitors ongoing communications.

AntiSpam

Message Inspector — Comprehensive email filtering software enables you to
proactively monitor, manage and, if necessary, block unauthorized “spam” email
communications.

Anti-virus

Message Inspector with AntiVirus — Anti-Virus sits at your Internet gateway to
provide protection against virus attacks from virus including in email
communications.

 

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Web Content Filtering

Web Inspector — Web Inspector helps organizations enforce Internet acceptable
use policies by monitoring Web sites that employees visit and blocking access to
inappropriate Web sites.

               All other terms and conditions set forth in the original
Agreement shall remain the same and shall continue in full force and effect.

               If this amendment is acceptable to you, please acknowledge your
agreement by signing both copies of this letter in the space provided below and
returning one fully executed copy in the envelope provided. The other copy
should be retained for your files.

Sincerely,

AVENTIS INC.

/s/ Vincent DeChellis
Name & Title Vincent DeChellis
Dir. E-Business, Business Unit Lead
9/13/04

         
 
  AGREED AND ACKNOWLEDGED:
 
       

  By   /s/ Ronald A. Woessner

       

  Name   Ronald A. Woessner

       

  Title   SVP

       

  Date   9/16/04