ResponseCode 421, Host not reachable

ResponseCode 421, Host not reachable

Can you explain about this error::

Your message did not reach some or all of the intended recipients.

 

      Subject:    RE:  Kumar here..!!! Kindly get in touch for all your IT Requirements, Call me : 00971566628384

      Sent: 12/30/2018 5:58 PM

 

The following recipient(s) cannot be reached:

 

       on 12/30/2018 5:58 PM

            550 5.4.6 Unusual sending activity detected. Please try after sometime. <a href=https://www.zoho.com/mail/help/usage-policy.html target=_blank>Learn more.</a>

 

Regards,
Kumar 
05666 28384