Error maildrop already locked

Email user in my office complaint because she could not download email from MDaemon server. Error message in outlook is:

Task '192.168.0.25 - Sending and Receiving' reported error (0x800CCC92) : 'Your e-mail server rejected your login. Verify your user name and password in your account properties. Under Tools, click E-mail accounts. The server responded: -ERR access denied'

After changing user password end retry to send and receive again this error is still occur. And then I check log mail POP in MDaemon server. The log status is fail with this error message in log file:

Thu 2010-07-01 08:11:17: Session 1437; child 4
Thu 2010-07-01 08:11:15: Accepting POP connection from [172.19.5.31 : 1062]
Thu 2010-07-01 08:11:15: --> +OK mydomain.com POP MDaemon 9.0.6 ready
Thu 2010-07-01 08:11:16: <-- USER john Thu 2010-07-01 08:11:16: --> +OK john... User ok
Thu 2010-07-01 08:11:17: <-- PASS ****** Thu 2010-07-01 08:11:17: --> -ERR maildrop already locked
Thu 2010-07-01 08:11:17: POP session terminated, (Bytes in/out: 20/145)

Then I check the mail box of the user and I found there are more than 10 emails in user mail box. One of the email sizes is more than 8MB. I tried to delete this email but I could not delete because the email file was locked. To delete this email I must restart MDaemon server first.