Exchange (SMTP) and Cisco PIX Firewall

There is an issue with sending and receiving SMTP mail from an Exchange Server through a Cisco PIX Firewall.

The solution is to be performed on the Cisco PIX, disable the MailGuard (SMTP fixup) feature on the PIX firewall, which is on by default. Run the following command form the PIX command line:

No fixup protocol smtp 25

Here is the step-by-step according to Microsoft.
To turn off the Mailguard feature of the PIX firewall:
1.Log on to the PIX firewall by establishing a telnet session or by using the console.
2.Type enable, and then press ENTER.
3.When you are prompted for your password, type your password, and then press ENTER.
4.Type configure terminal, and then press ENTER.
5.Type no fixup protocol smtp 25, and then press ENTER.
6.Type write memory, and then press ENTER.
7.Restart or reload the PIX firewall.

While the Cisco PIX firewall is generally a capable firewall, the MailGuard SMTP proxy feature has long been a source of problems, not just for Exchange, but for SMTP servers in general. However, it is an easy fix to resolve.

Here is the Microsoft article on this:
http://support.microsoft.com/default.aspx?scid=kb;en-us;320027