[Declude.JunkMail] Email with attachments kills the connection

[i360]

I have an interesting problem.   One of my clients does have a T-1 with AT&T.   When they send emails with no attachments it works fine, no problem at all but when they attach an a file and it does not have to be more than a few K's they get a timeout when sending emails.   If they send from any connection outside that T-1 everything works fine and if they send from any of the other T-1's they have with AT-T it works fine too.   ATT is pointing the finger at us of course.  The latest thing is that we don't have their reverse DNS set up right but the we don't check for it and if that was the case all the email should be rejected.   We even replace their firewall just to satisfy ATT with no luck.   Does anyone have any suggestions, I am at a loss.   Thanks,   Heimir