outgoing email with large attachment stuck in outgoing mail folder not able to delete

A mail with large attachment (movie) sticks in mine outgoing mail folder and I am not able to delete it.

I tried everything I could think of:

• deleting
• moving to other folder
• deleting only attachement
• deleting with no wifi
• restarted
• shut down and started

Please help because I cannot send other emails because this email blocks everything. Mostly the mail programme quits and start-up again.