sysObjectID.0 is not a MIB that is supported by the SMG appliance.
This issue has been addressed in Symantec Messaging Gateway version 10.0.1-2.
SNMP monitoring of the Symantec Messaging Gateway (SMG) is only supported based on the MIB files that are available from the Control Center.
For a list of the available MIB files, Please click on Administration> SNMP> MIBS to download MIBs files available. Page 721 in Symantec Messaging Gateway 9.5 Administration Guide (Configuring SNMP settings to monitor network devices).
Workaround for versions prior to 10.0.1-2
Please use the sysDescr.0 MIB to identify SMG installations until sysObjectID.0 is supported.