The message "571 Delivery not authorized, message refused (in reply to end of DATA command)" indicates that delivery of the email was not authorised and was therefore rejected. This typically occurs when the email server refuses to accept a message being delivered to its recipients.
The causes can vary, but it is often related to the sender not being authorised to send email to the recipient via the current server. This may 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 their IP address is not blocked (for example via MXTOOLBOX), and check whether there are any specific rules or restrictions on the email server preventing delivery. Contacting the system administrator of the receiving email server may also provide further information about why delivery was refused.
The error may also be caused by the content being flagged as harmful. Try sending a message without any attachments or links to see whether that makes a difference.