Email delivered, but an error is given to the sender

Email delivered, but an error is given to the sender

When I send a test email from one of my gmail accounts to my zoho account, the email arrives as expected, but then an error is sent to my gmail account.

Here is the contents of the email:

Address not found

Your message wasn't delivered to ********@*******.com because the address couldn't be found. Check for typos or unnecessary spaces and try again.
The response from the remote server was:

550 Unknown user