When this happens the user is unable to read his or her mail. PC programs may report this. So, for example, the lock file associated with johnsk6 is .johnsk6.pop . If the lock file is 0 bytes remove it. If it is not zero bytes move it aside and compress.
Chances are the user has read that mail already, but hang onto it just in case.
These usually come from Gail, or from one of the instructors for orientation. The problem is that a mailbox was left behind after a user account was expired. The fix is to change the ownership of the mailbox to the user. But, the mailbox may contain mail for the old user. Edit the mail box to remove those old messages.