I was using Outlook Express and had same problem. Turned out (as per Max Tech Support) that when someonehas Outlook Express AND Outlook installed on same machine and uses Outlook Express for email, it SOMETIMES goes to the Outlook outbox and sits there. They didn't know why. The solution was to switch to Outlook and remove Outlook Express.
Perhaps if you have Outlook Express on the same machine you might try de-installing it.
- R.