Many popular firewalls, such as Cisco PIX, Cisco ASA, Checkpoint, Raptor, and Watchguard models, will inspect and monitor SMTP traffic by default, although this is not made clear in their interfaces. This is known to interfere with SMTP traffic routed from hosted services, including hosted email filtering solutions. We recommend that this functionality is disabled in order to ensure continuity of email delivery.
On a Cisco device, run the command NO FIXUP PROTOCOL SMTP 25 from the CLI. On a Watchguard firewall, you need to create a normal TCP rule on port 25, rather than using the SMTP policy template.
Reference Points: http://support.microsoft.com/KB/320027