After upgrading Symantec Messaging Gateway (SMG) the MTA service fails to start. Port 25 is not accessible and Messaging Gateway (SMG) does not accept email.
"Error: Malformed MTA config XML: Can't use an undefined value as an ARRAY reference at /opt/Symantec/Brightmail/mta/lib/perl/SMS/MTA.pm line 1280."
The upgrade 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 resolution to this issue is to add at least one domain to the Sender Authentication domains list