I have several domains and email accounts on my server.
Some of these domains work. But on some the email doesn't get written in the maildir folder.
Also deleting and recreating in ISPConfig didn't help with these email addresses.
output of ehlo localhost:
250-AUTH LOGIN PLAIN
output of /var/log/mail.err:
Apr 24 18:19:54 pollux courierpop3login: LOGIN FAILED, ip=[::ffff:188.8.131.52]
Apr 24 18:20:20 pollux last message repeated 23 times
Thanks for your help!