If a message doesn't get sent properly the first time, it tends to get stuck in the Outlook Outbox. To attempt to send the message again, you have to open the message from your outbox (e.g. double click) and then hit "send." Not exactly intuitive...
For some reason the "send/receive" button doesn't always work as well.
Update: I've recently had this happen again, and no combination of opening/sending/closing outlook/etc. seemed to work for one out of two email accounts that I was accessing via Outlook ( 1 IMAP - broken sending, 1 POP3+PST - worked ). I ended up recreating the IMAP account settings. YMMV.