I have an exchange 2000 server. I receive emails into the local queue that get stuck and the queue goes in to a retry mode. It only happens to a few customers that email us. One customer it only happens to email that he replies back to us with. I can see the email in the queue and the only message is that is in going to retry... after 16 hours the sender will receive a undeliverable message. I send and receive other email with out problem.
I have tried to telent to their email server on port 25 without issue.
If the customer with the reply issue emails me a new message I receive it without issues.