Originally Posted by treeman
Had to set the MTU on the network card to match the MTU on the modem/router. Max MTU on modem was lower then network card, after setting network card MTU to match or lower everything started to come through.
I just had to register here to say "Thank you!" for finding out that the MTU was the problem here, and posting to let everyone know about it.
I was suffering the exact same problem, but I was using qmail at first. I noticed multiple qmail-queue processes were sometimes just hanging for a while before dying, but not a single error was being logged. The strange thing was occassionally some mail was coming through, but typically mail with attachments were problematic.
I then ditched qmail and changed over to Postfix, but I still suffered the same problem -- however Postfix at least wrote the "timeout after DATA" message to the log. That's when I googled and found this post.
I first thought it unlikely that the MTU would cause such a bizarre problem, but I was pretty much willing to try anything at that point. Indeed, my router was set to use an MTU of 1492 while the mail server was using an MTU of 1500. One simple 'ifconfig eth0 mtu 1492' command on the mail server, and like magic... mail was working perfectly fine again.
I don't know how you found the correlation between the data timeouts and the MTU, but thank you for posting the answer here. You definitely saved me some time (and a few grey hairs).