[otrs] POP3 locks

Every time my OTRS connects to my pop server, the account is locked, then each time OTRS attempts to connect, or if we try to log in via web mail, the connection is refused, telling us to check auth details etc. When telnetting into the server, we log on and it says mailbox locked. It seems the only way we can unlock it is to delete the mail box. Any ideas how we can resolve this one? Quite important. Thanks Mike Hayward