Returned Email message

Only getting this error message from one person I email. Is it his settings or mine?

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.

j INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :Please turn on SMTP Authentication in your mail client.
sender-of-o51.zoho.com [135.84.80.216]:21117 is not permitted to relay
through this server without authentication.