internet mail does not recognize user

I was thrown in this mess. NT4, exchange 5.5.
Consultant left after bringing Email in-house, from DSL ISP, and now the only person who CANNOT send/receive email is, of cause, the BOSS. In outlook, He keeps getting the pop-up box of hell, enter username and password.I tried to duplicate the settings of POP3and SMTP from working machines, to no good end. Help.