We have a single Exchange 2003 Enterprise server with about 150 users. Lately - over the last month some emails have begun to languish in the queue until delivery failure. Other emails from the same senders to the same recipients go through, and internal emails are sent as well.
During troubleshooting, I was able to do a packet trace and see that for a normal email, after about 4 fragments, we begin to receive TCP ACKs from the other server. With the problem emails, we also receive the ACKs, but the RST flag is set as well. The TCP window is also widening, so it appears the external server really is ready for more.
It seems that if the user forwards an email that has this issue, the forwarded message also has the issue, and if they put the same content into a new email, it goes through. This gives us a work-around, but I would like to know if this is a known issue?