The cause of this error can be hard to determine, because what we get back from Google is simply “not-authorized.” That doesn’t say much, and it could mean anything from entering the wrong credentials to account locked-out.
If you are certain that your username, domain, and password are correct, try following the advice on this page from Google. Today we were troubleshooting a problem with one user not being able to sign up. Others were signing up just fine, and this user could connect to GMail and other services with the account. Following the “Display Unlock Captcha” procedure listed on that page freed the account and he was then able to sign in.