The error message "571 Delivery not authorized, message refused (in reply to end of DATA command)" indicates that the delivery of the email message is not authorized and, therefore, was rejected. This typically occurs when the email server refuses to accept a message that is attempting to be delivered to its recipients.
The reasons for this can vary, but it is often related to the sender not having the authorization to send email to the recipient through the specific server. This could be due to authentication issues, the sender's IP address being blacklisted, or other security measures preventing the message from being delivered.
To resolve this, the sender should double-check their authentication credentials, ensure that their IP address is not blocked (check with MXTOOLBOX), and check for any specific rules or restrictions on the email server that may be hindering the delivery. Contacting the system administrator of the receiving email server may also provide additional information on why the delivery was denied.