If you suspect the content filter on a Cisco PIX is interfering with mail delivery, you can disable it as follows.
By default, the PIX Firewall inspects port 25 connections for SMTP traffic. If you have SMTP servers using ports other than port 25, you must use the fixup protocol smtp port-number command to have the PIX Firewall inspect these other ports for SMTP traffic. The syntax of the fixup protocol smtp command is as follows:
fixup protocol smtp port [-port ]where port[-port] is a single port or port range that the PIX Firewall will inspect for SMTP connections.
Use the no form of the command to disable the inspection of traffic on the indicated port for SMTP connections. If the fixup protocol smtp command is not enabled for a given port, then potential mail server vulnerabilities are exposed.
Microsoft has a knowledge base article which explains this: