There is a need to know more about "MTA scheduler concurrency" setting in SMTP Advanced Options, also how to configure it accordingly to the environment.
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 at default value of "2". There might be further enhancements to this functionality in future releases. Please subscribe to this article to be notified of any updates.