Sometimes the service POP on your server may not fucntion properly. The reason are many, still you can try the following even if the service is not starting or coming up after you have tried from your WHM, at “Main >> Restart Services >> POP3 Server”
You may also try “/etc/rc.d/init.d/cpanel restart” and “/etc/rc.d/init.d/courier-imap restart”.
If any of the above should not work for you, then you will need to check the file at “/usr/lib/courier-imap/etc/pop3d”. Place a value for “MAXPERIP” and restart courier-imap and it should work for you.