I was at a client's place today and she had Outlook 2003. I set her up with POP3 and tried to do a send/receive and it said "send/receive finished," but nothing happened. The test message I tried to send was still in the outbox, and the inbox was empty.
This problem also made it impossible for her to run Activesync with her Windows Mobile PDA. Whenever I tried to set it up, a message box would come up and tell me to install Outlook, run it once, and then run the synchronization wizard again.
Well, lo and behold, this was a known issue with Outlook 2003. A simple upgrade to Office 2003 SP3 fixed the problem.
Only took me 3 hours to figure it out!