OS X 10.11 El Capitan Upgrade : Mail Stuck in Sending Loop

Several clients have now reported the same issue after upgrading to OS X 10.11, Mail is immediately stuck in a loop sending emails. A blank email window flashes up and disappears almost instantly, Mail reports it is sending (in this case) 1 out of 4 emails. To fix the issue I Quit (or Force Quit) Mail, then browsed to the following folder :

~/Library/Mail/V3/Mailboxes (Note the V3 folder which arrives with El Capitan)

Delete the mailbox “Outbox.mbox” and reopen Mail.