Incoming messages are not equally distributed to multiple downstream servers when a domain has multiple destination routings specified with the same MTA preference in following configuration areas in Symantec Messaging Gateway (SMG) GUI:
1. Protocols -> Domains -> Delivery
2. Content -> DLP Connect -> Filtering
3. Administration-> Hosts -> Configuration -> <Hostname> -> SMTP -> Inbound -> Inbound Local Mail Delivery
4. Administration-> Hosts -> Configuration -> <Hostname> -> SMTP -> Outbound -> Outbound Non-Local Mail Delivery
However, SMG administration guide might suggest to type the same number for each host in the preference to perform load balancing, e.g:
"Type a number between 1 and 100, inclusive, under Preference, to indicate the order in which hosts should be used. Lower-numbered hosts are used first. To perform load balancing, type the same number for each host."
Symantec Messaging Gateway (SMG) does not provide round robin type of load balancing for downstream servers when multiple destination hosts have the same MTA Preference assigned.
SMG will utilize the healthy route as much as it can before it tries the next one with equal preference value. Therefore, there is no guarantee for customer for equal distribution of messages to downstream mail servers. SMG is working as designed.
It is suggested to use a DNS server with round robin option enabled or external load balancers for effective distribution of load.
Symantec is aware of this issue and will update this document when a solution becomes available. It is not necessary to log a support case on this issue. Please subscribe to this article to be notified of any updates.