MacMail - messages stay in Outbox but still send

For the past week messages are staying in the Outbox. They are sending OK and show in Sent messages. I have both Outlook and Gmail accounts and it's only the messages being sent from my Gmail address which are affected. When I try and delete or move them from the Outbox to any other folder just to get rid of them they initially disappear then come back a few minutes later. Everything looks fine in my individual Outlook and Gmail on my web accounts.

I've seen a post on here about "sandboxes" but unfortunately it was beyond my basic Mac comprehension!