A common problem for many Outlook users is that new emails seem to be stuck in the outbox. To get the emails sent, you then have to press Send/Receive (F9) button. Most users (me included) prefer to have the emails delivered as soon as we press Send, not collect them and send all later on.
Here are some things to try if your sent mail is getting stuck in the outbox (depending on our Outlook version, these may vary):