Mail was working fine till yesterday and today senders to this address are getting this message. 

INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :(sender163-mail.zoho.com) [74.201.84.163]:24429 is currently not permitted

                                                to relay through this server. Perhaps you have not logged into the pop/imap

                                                server in the last 30 minutes or do not have SMTP Authentication turned on

                                                in your email client.