Hmmm could be difficult to track down - quite a few options.
Outbox is where messages are supposed to go whilst they are queued to be sent.
The question is why yours are not leaving the Outbox - here are some things to check for a start:
1) Have you inadvertently gone into "working offline" mode? On the "File" menu there should not be a tick against "Work Offline". If there is, click the option again to remove the tick.
2) Click on "Tools / Accounts" ... is your Mail account listed there? Select it and click "Properties" ... make sure that "Include this account when receiving mail or synchronising" is ticked.
3) Click on "Tools / Send and Receive / Send All" ... what happens? Any errors reported?
RC