Do you have AV actively scanning incoming and outgoing email? This can be the
culprit. Disable AV email integration and try again.
Do you have your send/receive poll interval too short? 10 minutes is what
seems to be optimal. If your send/receive poll is set to a very short
interval AND you have AV actively scanning, you can see the problem this may
cause. In addition to not sending you could have multiple sends.
Those two would be my first guess. Change your send/receive poll to 10
minutes or longer and disable email scanning.
For those emails currently stuck in your outbox, try the following:
http://www.howto-outlook.com/faq/messagestuckinoutbox.htm