Hi!

Set Outlook to Work Offline, click File > Work Offline.
Close Outlook.

Open the Windows Task Manager. You can do this by right clicking the taskbar and choosing "Start Task Manager" from the pop-up menu or by pressing CTRL + SHIFT + ESC.

Then switch to the Processes tab and verify that no outlook.exe process is there. If there is one, select it and click End Process.

Start Outlook again.

Go to the Outbox and open a hanging message.

Now you can either delete the stuck message or move it to the Drafts folder and remove the attachment if it is too big in size and this is the root of the problem.

Then you can try to send the message again.

Bring Outlook back on-line by clicking the "Work Offline" button.

Click Send/Receive and see if the message is gone.

Bye!

Souriane