I am working with an ASA 5520 with a SPAM appliance located within the DMZ. Not all smtp connections are being corrupted by the inspect esmtp setting, just a few. It was discovered that those few sites that are connecting to the SPAM appliance traverse 2 additional firewalls (1 ASA and 1 PIX), *before* their smtp traffic hits the Internet to continue on to our DMZ.
Why would this be the case? Is it due to passing through two additional firewalls that may be adjusting the headers (static NAT, etc.)?
If we are not comfortable turning off the inspect esmtp setting, is it possible to write a specific policy that would include these few sites MX records? If so, how might that be done?
You could create an access list
that matches specific server ip addresses and put it under the policy map and inspect esmtp on it
access-l esmtp-acl deny tcp any eq 25
access-l esmtp-acl perm tcp any any eq 25
match access-l esmtp-acl
I hope it helps.