Screen shot 2014-09-09 at 10.05.42 AMI ran into this issue yesterday when trying to add a new non-gmail address to my Gmail account:

You must send through SMTP servers when you send as However, this functionality is not available for your account. Please contact your domain administrator.

I used the Chat help option from Gmail and was helped by Dimitar who suggested the following fix:

Dimitar B:  Gmail
Dimitar B: Advanced settings
Dimitar B: Under “Allow per-user outbound gateways” – Disable the checkbox and re-enable it again
Dimitar B: This should activate this setting although the downside of this is that it could take up to 24 hours to propagate in full