There are two files that you need to be concerned with, imta.cnf and job_controller.cnf. In job_controller.cnf you define the pools and the maximum number of jobs that can be run at a given time in those pools. In the imta.cnf file you define what pool a channel uses and the maximum number of processes the channel can run in that pool at a given time.

In large deployments machines are dedicated to a specific role. On a MTA-IN machine one would configure more jobs to the tcp_intranet channel and thus deliver mail faster to your mail stores.

In large sites they may wish to adjust MAX_MESSAGES. The Job Controller keeps information about messages in an in-memory structure. In the event that a large backlog builds, it may need to limit the size of this structure. If the number of messages in the backlog exceeds the parameter specified here, information about subsequent messages is not kept in memory. This parameter is set in the global section of the job_controller.cnf file, this variable is not present by default. The default value is 100000. For an in depth explanation about MAX_MESSAGSES see my personal website, http://ims.balius.com/ and look for the topic max messages.