Reverse DNS is not enabled in the Symantec Messaging Gateway (SMG).
For versions of SMG before 9.5.x, mail flow is not affected if the lookup passes or fails.
Since SMG 9.5.x, DNS Validation has been introduced. Administrators can specify an action for failed reverse lookup.
To find these settings:
Unchecking "Enable reverse DNS Lookup" results in the MTA ceasing to perform reverse DNS lookups for each connection. This results in a nominal performance gain in most environments, but may result in a more pronounced performance gain in environments which have communications issues between the scanner and the name server which services the DNS lookup requests.