Mail to some domains not delivered

Mail to some domains not delivered

Hi.

When I send an email to a user in a particular domain it fails. If I send to a gmail account it succeeds. Can you please help me?

The message I go is below:

This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

sunderam@sunderam.net INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :User unknown



Received:from mail.zoho.com by mx.zohomail.com
    with SMTP id 1503984503585544.6365207616927; Mon, 28 Aug 2017 22:28:23 -0700 (PDT)