Apple mail is looping email
I send an email and it started looping eternally.
It keeps popping up. Can't close the email, can't go to the outbox to delete the mail. It keeps popping up.
Contacted provider, no problems at their end.
Deleted all my accounts in mail, but the email is stil teher and keeps popping up.
Can't do anything in Apple mail (restarted programm several times and retarted macbook pro (high sierra 10.13.3) several times....didn't help.
How can I stop this loop and use mail again?
Regards,
Brtz
MacBook Pro, macOS High Sierra (10.13.3)