If you are reading this post then chances are you have received an error message that looks like this
There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.
from an email user that tried to email a message to your organization. More specifically, the sender was trying to send an attachment, and you are probably witnessing an intermittent error.
In my experience this is caused by one of two things. Either you have a firewall that is doing application level content inspection (Cisco Pix, Checkpoint, Watchdog, etc…), or if you look through your smtp logs you will see errors to the effect of command too long, or invalid command.
First things first, disable the smtp proxy on your firewall. Why? Because most firewalls are very basic and don’t understand/can’t process the EHLO verbs correctly. This is a constant problem with the Cisco Pix, other firewalls are hit and miss.