SNMP is enabled and the Host Access list includes the Fully Qualified Domain Name (FQDN) of hosts allowed to connect to the SNMP agent running on Symantec Messaging Gateway (SMG).
After rebooting SMG, the agent does not respond to SNMP queries anymore.
The problem is resolved in SMG 10.5.
If you can not upgrade immediately, as a workaround please use IP addresses instead of FQDN in Host Access entries for SNMP.
If you must use a FQDN, disable and re-enable SNMP after rebooting SMG.