POP3 Email Error
I’ve just spent a heap of time trying to figure out what was going wrong with our email server.
The only error was -ERR (try searching Google for that!). The server was popa3d.
When I tried telnetting into the server (port 110) it would behave normally unless I put in the correct password, in which case it would just spit out -ERR. So after getting nowhere with Google I gave up and installed Dovecot instead… STILL GOT ERRORS, however they were not valid mbox format, clever me figured I must have messed up the config so I spent hours trying to workout why it wouldn’t work for this one user… turns out his mbox was corrupted 
Easy fix though sadly he isn’t going to be getting his old mail.