Outgoing mail not dispatched but stuck in outbox

In addition to the legit address (that you carefully checked, right? Sometimes mistyped addresses keep reappearing), there might be something else that you don't see.

I suggest making sure all three To, CC and BCC fields are totally clear, then put back the single address you want to use.

You could try saving the contents of the email if it's text. I then would suggest you remove the email account from mail and re-add it to Mail.