The correct format for the mail account username is specified by the
provider of your mail account. Some require you to use the full email
address (
[email protected]) while others only want the part in
front of the @ symbol. Try it both ways if you can't find your
provider's online setup instructions.
If you continue to get that prompt for username and password,
click Cancel. It should then show an error message. Right-click
on your error message, copy, then paste it into a reply here.