sync wrong email address

Version: 8.0.2016 (bab6791)

I start sending two emails. And now I get this problem:

[SMTP] Die Nachricht (Der E-Mail-Empfänger wurde vom Server nicht akzeptiert.) kann nicht gesendet werden

This may be. But how I delete this mail, because emClient tries to sent every ten minutes since 2 days.

Go to Menu > Settings > General and make sure that Show Local Folders is selected. Now go to the Local Folder Outbox and delete the message.