While trying to add another email addy to your gmail account when the new email addy is on your domain with google apps, you get various error messages like
Authentication failed. Please check your username/password.
[Server response: 535-5.7.8 Username and Password not accepted. Learn more at 535 5.7.8 https://support.google.com/mail/answer/14257 hq6sm1707962igc.11 – gsmtp code(535) ]
This implies the username/password combo is wrong but it is not.
These settings worked for me:
SMTP Server: smtp.gmail.com
The system will then send you the verification email.
If that alone does not work, click through at this link then try the above settings again: https://accounts.google.com/b/1/DisplayUnlockCaptcha