Cisco firewalls have a program called SMTP Fix Up. This program is known to cause problems with mail delivery, and we recommend disabling it. Below is an article from Microsoft that covers the problems with SMTP Fix Up:
To disable SMTP Fix Up:
- Establish a Telnet Session to log on to the Cisco PIX firewall. Alternatively, use the console to log on to the Cisco PIX firewall.
- Enter the enable command.
- Enter your Password when prompted.
- Enter the following commands in sequence:
- configure terminal
- no fixup protocol smtp 25
- write memory
- Restart the Cisco PIX/ASA Firewall.