The following non-delivery report:

Your message can't be delivered because delivery to this address is restricted
Delivery is restricted

means that there are delivery restrictions in place for the recipient address.

The Diagnostic information for administrators section also contains the following error:

#550 5.7.1 RESOLVER.RST.NotAuthorized; not authorized ##
Not authorized

Usually, such a bounce-back is generated when the email is sent to a Distribution List with delivery restrictions in place. Delivery restrictions settings can be changed under Services > Distribution Lists > the distribution list name > Delivery Restrictions:

Delivery restrictions

To resolve the issue:

  • contact the recipient mail administrator to see if it's possible to include the sender's address to the allow list
  • or send the message from the allowed email address.