Outlook 2007 IMAP vs Outlook 2003 - Can't send mail on 2007

Any machine I take with Outlook 2003 and configure for IMAP on my mail server
sends and receives with no issues. Even unchecking "server requires login"
the client still sends and receives. Basically I can't break it if I tried.

No matter what combination of options I use in Outlook 2007 however, I
absolutely cannot get the client to send mail. Are there any specific
differences between the 2 versions that would cause this kind of problem? XP
and Vista are irrelevant because Outlook 2003 sends/receives just on both OS'
while Outlook 2007 can't send on either. The Outlook logs didn't give much
info. The mail server is Exchange 2003 on Windows Server 2003 domain.

Any ideas or tools to try and figure this out?? Thank you!