Problem: There has been an error transferring your mail. I said PASS and the POP server said
-ERR: Unable to lock Maildrop
OR
-ERR: Maildrop lock busy.
Is another session running?

Solution:

This will happen if a connection made to the the POP server has been broken abnormally. The mailbox will usually reset itself automatically in a few minutes (30 minutes usually). This can also happen if another connection is currently open to your mailbox.