(FAQ) What is the purpose of LCK files?
My users can't POP mail. The error in the log is 'Maildrop already locked' and there is a .lck file with their username on the MDaemon server.
During a POP session there is a lock file that is put on MDaemon so that only one client can POP at a time. After the POP session is over, the lock file is removed from the mail server to allow the next POP session. If you're checking your mail again during the first POP session, then this error will occur. Sometimes the lck file doesn't get removed/deleted properly and you'll have to restart MDaemon (after the session is over) to remove it or go into the MDaemon\LockFiles directory and remove it manually.
The best way to avoid this, is to not send/receive so quickly. If you click it twice in a row, you may get this error.