You have found that some inbound message deliveries to the Messaging Gateway are rejected with the following response:
554 5.7.1 Delivery not authorized
Release : 10.7.3
Component : Default-Sym
The rejection response "554 5.7.1 Delivery not authorized" is due to the configurable reverse Domain Name System (DNS) validation feature of the Messaging Gateway with a default rejection message.
The feature is documented in the following document:
The rejection indicates that the reverse DNS query for the connecting server failed. Further detail on which of the revers DNS validation checks can be found in the Message Audit Logs.
DNS will need to be investigated to determine why the check failed and corrective steps would need to be done one the DNS records depending on the results of the investigation.