Hi, First of all thanks very much this is very helpful!
I have followed your directions and when sending a test mail this is the response I received:
Final-Recipient: rfc822; xxxxgmail.com
Remote-MTA: dns; smtp.zoho.com (188.8.131.52, the relay for the domain.)
Diagnostic-Code: smtp; 553 Relaying disallowed as xxxx <email@example.com>
Last-Attempt-Date: Wed, 08 Feb 2017 22:35:14 -0800 (PST)