Users will occasionally get a dialogue box asking for a username and password. Users often get confused and will either input an incorrect username or try entering their email address in the username box.
The username should be entered in this field, NOT the email address.
First part of email address ≠ username
Traditionally the username of a user on a domain is the first part of their email address, e.g. for ecavazos@evolveip.net the username just happens to be ecavazos. However, this isn't always the case. Your best practice would be to verify the user's account name in EAC (or EMC for Exchange 2010) by matching the email address that Outlook is referencing (see the yellow highlight in the screenshot below) to the correct account name. This is doubly important when a user has multiple internal and/or external email addresses, because separate accounts mean separate usernames. Finally, if the user is getting multiple dialogue boxes asking for credentials for different email addresses, you will have to repeat this step for every instance.
If you have verified the domain and username are working then the user is probably entering in the wrong password. Have them test with OWA. If it fails there, the password is wrong. You can reset it with Active Directory or HEX.
