The article describes more about "MTA scheduler concurrency" setting in SMTP Advanced Options and how to configure it accordingly to the environment.
Messaging Gateway
This option might allow for tuning MTA resources based on CPU resources available. However, Symantec does not recommend to modify this option. Please leave this option as default value of "2". There might be further enhancements to this functionality in future releases. Please subscribe this article to be notified of any updates.