can't see outbox in Mail macOS 26.0.1

There's a piece of mail stuck in my Outbox (which I can't see for some reason) that keeps trying to send itself, whereupon it gives me an error message. Then it keeps trying to resend the mail at intervals. I can't get it to stop and eventually have to Force Quit Mail. How do I fix this? If I could see the Outbox I'd just delete the mail, but I can't.