When attempting to send a message which is too large for your mail server, it may become stuck in the outbox, unable to delete it, because Outlook says it has already started to send the message.
If this happened to you, there are two things to try...

Try setting Outlook in offline mode:

In the menu, click File, then Offline

Wait about a minute or so, then close and reopen Outlook

If everything went well, you should now be able to delete the message from the outbox, or at least, move it to another folder before attempting to delete it.

When this worked, disable offline mode: click on File in the menu, then again Offline

Finally, restart Outlook and everything should be well again.

If   ...