Getting immediate rejection when replying to a specific person.

Getting immediate rejection when replying to a specific person.

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.

ERROR_CODE :550, ERROR_CODE :5.7.1 Access denied

I am able to send to other addresses without any issue. Rejection appears to be from Zoho mail server, not from the recipient's mail server.