So would it likely to assume that Username and password are incorrect ?
If the computer receives email, then the username and password were likely correct. Outlook's account setup wizard will only ask for the information one time by default.
If this system has never been able to send email, what is likely incorrect is the email server configuration details.
If it once sent email, then mostlikely Outlook had a connection problem once and popped up the credentials box, then the user put in the wrong info.
This seems VERY likely at this point!
Probably entered the Windows password he uses to login.
Probably his personal email password from home.