Can't receive emails but can send them

I had posted before re getting a Windows Security pop down when I try to
receive emails in Windows mail. I turned off my email security in AVG, as
suggested but it made no difference. I intermittently can receive emails but
most time not.
The error message I get is

Account: 'mail.bigpond.com', Server: 'mail.bigpond.com', Protocol: POP3,
Server Response: '-ERR [IN-USE] account is locked by another session or for
maintenance, try again.', Port: 110, Secure(SSL): No, Server Error:
0x800CCC90, Error Number: 0x800CCC92

I can't figure out what else it could be. Anyone have any suggestions?