We are in the process of setting up a spam filter (SAVASM). One change we are making is to push incoming email on port 25 through our spam filter/server but have users actually send their email on a different port. I am attempting to make this happen by using port address translation to send port 25 traffic to the SAVASM server IP.

As a step in making this change I setup port translation without actually changing the IP addresses. The NAT rules for the email server went from one Static NAT rule with no port specified, to multiple Static NAT rules each with a port or group matching the Access Rules for that server (smtp, pop3, http, https, and some other custom ports).

The problem we are running into is confusing. Some outgoing mail through this server is failing when the router has the multiple NAT rules with port translation settings. Email goes through fine FROM our email to our internal accounts and to Gmail. However email fails when FROM our client's email address TO our client's email or their personal Comcast. The only situation that worked for them was if they changed FROM to Comcast and then messages went through fine to both Comcast and the client's accounts. Switching back to regular Static NAT rule everything then worked for them.

Does anyone have a clue as to what might be going on? We are on a Cisco ASA 5500 box.