It seems like an authentication error; have the user give you their username & password, then see if it works for you (telnet into port 25; "Helo <x>"; "user robert"; "pass <pass>"). If the authentication fails, it's a bad password, if it succeeds, it's PEBKAC
(on the user's side).
The user might actually be getting disconnected from the server, but you need to find out what the exact error is. A disconnection error is much different than an authentication error.