I configured the email listener with concurrent threads value set to 5. What would happen if there are large number of emails received at the same time? I believe it's going to queue and process whenever the thread is free.
How do we determine the ideal number of concurrent threads for 200 emails/hour?
200 emails per hour does not sound like much of a load unless the Email Service activity is forced to do work that is so lengthy the listener threads on a given node are completely consumed.
Multiple nodes within a cluster can be configured to support email processing.
Plus case logic can be designed to detect being processed in the context of an Email Service and/or received email. If so, Queue-For-Agent can be invoked to allow processing to continue using a Batch requestor.