How do I clear my outbox? Emails stuck in "sending" mode

I tried sending an email with two short videos and a small image attachment, it said that MailDrop couldn't send it and to try again later. I then tried sending the email with only one video, it said the same thing. Then I tried compressing the videos and sending them, it said the same thing. I tried sending the message without any attachments, then I tried a few other variants... Now the status bar at the bottom says it is "Sending 8 of 8 Messages" and they're all stuck in the Outbox. Mail won't even let me close the app now because it is still trying to send them all. What do I do??