Mail

Hi,

Have you tried restarting the Mac and deleting? If no luck, try booting into Safe Mode: How to use safe mode on your Mac - Apple Support and see if the issue persists there. You may have to just wait awhile for it to send.

Bardium wrote:

Mail message with too large attachment is stuck in outbox but won't let me delete.

quite Mail, relaunch it holding the shift key...and try again

Mail User Guide for Mac - Apple Support