POP failed

Do you know when cPanel was installed on the server, how long ago?

If it was in the last couple of months, chances are that it will be using the more recent mail setup of maildir-formatted mailboxes and it should be using Courier pop and imap servers.

If so, you might run into some problems trying to restart pop from WHM. I previously noticed that even after Courier was up and running, restarting pop through WHM killed off Courier and started cppop, which is the old cPanel proprietory pop server. I haven't tried doing the same for ages and so I'm not sure if this is still an issue. Hwever if your server should be running Courier but is instead trying to run cppop, you might encounter problems.

You'd have to first find out if your server should be running Courier imap and pop and what format the mailboxes are in. If you're sure that the mailboxes are in maildir format and you're sure that you should be using Courier, try /scripts/courierup --force.

If you're not sure, go to /home/username/mail/example.com/mailuser/ and describe the file structure you see there and also check what is listed, mail-wise, on the service status screen in WHM.