How do I resolve a stuck outgoing email with videos in the Mail app on my MacBook Air?

I tried sending an eMail with videos on April 3 (20 days ago) from my MacBook Air using the MAIL app... it is still there, stuck and making me crazy... it never left, it makes my MAIL app close automatically and I get the colors circle thing while loading/thinking and then it closes... I tried deleting it by sliding and then update, and by clicking on the little trash can and it doesn't disappear, if I open my email somewhere else it is not there, it´s only on my MacBook and I can´t use my work emailk so I really need help. Thanks

[Re-Titled by Moderator]