I had a problem similar to this a week ago with a person who could receive mail but not send mail with 2003. For some reason when I changed the email type from Rich Text to HTML the emails could be sent without any problems. I was not using RCP-HTTP though. Just a thought…..
I have a strange issue with one user. I have Exchange 2003 on Windows 2003, latest service packs and updates. One user has a workstation and a laptop (2 computers). Both connect via RPC/HTTP across the public Internet. Both are configured identically for Outlook (same profile and account). On the desktop computer he is able to receive and send emails just fine. On the notebook, ON THE EXACT SAME NETWORK SEGMENT, he can receive email but when he tries to send email, it just sits there in the Outbox and does not get sent. Apparently the notebook was working just fine, then quit sending today all of a sudden for no obvious reason.
Any clues what I should look for?