@mossman Since mails can be sent and received, it does not seem to be a block problem.
I just emptied the trash on a trial basis. 1 mail is left (test b1). It was a test mail. It cannot be deleted in any way. Here is a screenshot:
papierkorb.jpg
EDIT: I could empty the trash now by reset the filters. After reset of trash filter the trash bin increased from 1 item to 64 again. Then I had to delete them again.
IMO the problem is caused by the filters somehow.