1.) If you have Anti virus set to scan emails, turn it off, see if this problem stops.
2.) Go into the Outlook Options, then the Advanced option and under SEND AND RECEIVE is an option that says SEND IMMEDIATELY WHEN CONNECTED. Check the box to activate this option.
It doesn't happen too often but when a message is stuck in the Outbox it can be a real pain. Below you'll find three methods to get the message out of the Outbox.
Open the message from the Outbox and close it again. Now select the message and press DELETE
Put Outlook in offline mode. To do this go to File-> Work Offline. Now restart Outlook and see if you can delete it (you can try this in combination with method 1). Don't forget to put Outlook back On-line again.
Create a new pst-file; File-> New-> Outlook Data File...
Set this pst-file as your default delivery location in your Account settings; Tools-> E-mail Accounts-> button Next
When you now restart Outlook your original pst-file will show up as the secondary set of folders where should easily be able to remove the file from or even the complete Outbox folder
Set the original pst-file as the default delivery location again and restart Outlook; the Outbox folder will be recreated and will be empty.
Use the Microsoft MDBVU32.EXE tool to forcefully clear the Outbox. The instructions for this are the same for when removing a stuck read receipt but instead of choosing "MDB-> Open Root Folder" choose "MDB-> Open IPM Outbox".