After upgrading Symantec Messaging Gateway (SMG) from version 10.6.4 to version 10.6.5 or later, the MTA service fails to start. Port 25 is not accessible and Messaging Gateway (SMG) does not accept email.
The upgrade from 10.6.4 in some rare instances, removes the domains from the Domain Authentication field in (Spam -> Sender Authentication).
(NOTE: This issue may occur whether or not you have only the following domains selected)
The recommended upgrade procedure from the Messaging Gateway 10.6.5-1 Release Notes say to note the information on the Sender Authentication page prior to upgrade and confirm the settings are correct after completing the upgrade.