Bounced email - not understanding the reason

Bounced email - not understanding the reason

I tried sending a short email with a PDF attachment this morning and it bounced. The information provided is as follows:

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.

<sent to address> 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]:21097 is not permitted to relay through this server without authentication.


I am quite certain that I entered the recipient's email address properly. Assuming that I did, what could be the cause of this error?