Mac Mail has stuck messages in Outbox

Apple – Support – Discussions – Mail stuck in OUTBOX …

The problem is a corrupt Outbox. I read about this on
the ATT trouble shooting site. Delete the Outbox and
mail will create a new one. This solved my problem with
email sitting the Outbox forever and never being sent.
To be safe, I dragged my Outbox to a different location,
then started mail. Problem solved!

The Outbox is found: Users/YourName/Library/Mail/Mailboxes/Outbox.mbox