EXCHANGE SERVER PROBLEM

MY CLIENTS AND SERVER CAN SEND MAIL BUT CANNOT RECEIVE MAIL WITHIN MY INTRANET.
MAILS HAVE BEEN CONFIRMED TO BE ON THE SERVER THRU THE IMS PROPERTIES FOLDER.
GIVES ERROR MESSAGE:”There was a problem logging onto your mail server. Your User Name was rejected. Account: ‘Administrator’, Server: ‘10.0.0.1’, Protocol: POP3, Server Response: ‘-ERR bad username’, Port: 110, Secure(SSL): No, Server Error: 0x800CCC90, Error Number: 0x800CCC91″. HAVE CONFIG. THE OUTLOOK AND EXPRESS TO USE EXCHANGE SERVER.
WHAT DO I NEED TO DO- SERVER AND CLIENT.