I have been having a problem recently where some of our mail is not getting to clients. I looked into the Message queue on my SBS2k3 server and noticed that several recipients are showing up in the queue with a state of "Retry". When I highlight the recipient, the additional queue information says "An SMTP protocol error occured". How can I find out more specifically what error occured, and how could I then go about correcting it?
You can right click or double click the message (can't remember exactly) and you might get more information. Also, if you have SMTP logging enabled you can check that. You can also telnet to the SMTP server on port 25 and go through parts of the process to see if anything comes up.
Also, see this answer to a question I asked for some more advice.