Upgrade to the 10.6.5 version of Symantec Messaging Gateway (SMG) is not visible under "Version(s) available for update" found at Administration > Version > Updates.
The current SMG version is not 10.6.4-3.
In order to upgrade the SMG to version 10.6.5, the SMG needs to be upgraded to 10.6.4-3 first.
Once the SMG is on 10.6.4-3, the 10.6.5 update should be available when clicking the "Check for Updates" button under Administration > Version > Updates.