The pop file does not exit the mailbox cleanly. It leaves the .lock file.
I have asked around, but no one knows why the lock file is not being removed. Everyone agrees though with the fact that the script is not existing cleanly.
For some reason when the wp-mail.php script is run, the POP server leaves a lock file behind and ask if there is something that could be causing this. I installed WordPress on other servers and they all experienced the same problem. I think this is happening across all cPanel servers.