emailing between two zoho accounts

emailing between two zoho accounts

I set up an email account for one entity which is working fine. I changed the mx addresses at godaddy and all was fine.There are two email addresses in that account.
I then set up another account for a subdivision we created using a different email address.
I did the same thing at godaddy and then ran into some issues.

Two things happened. I was not able to send emails from the new to the original account and the original account was unable to receive any emails from anywhere. 

In order to restore functionality, I couldn't figure out the problem so I deleted the second account and the original started working again.

What was I doing wrong?
Is there a problem that the two domains are registered under the same account? 

The message that kept coming back was:

Technical details of permanent failure: 

Google tried to deliver your message, but it was rejected by the server for the recipient domain rtggroupusa.com by mx.zoho.com. [165.254.167.50].

The error that the other server returned was:

553 Relaying disallowed