Max Send Jobs
This setting controls the number of maximum mail jobs that can be sent at once. In general, the default setting will work fine.
However, if you are running a site that has a very large number of one-off messages, such as autoresponders, or many subscriptions, you may want to increase the number.
You can tell if you need to increase this number by looking at the statistics page. If your "number of jobs" number on the statistics page is consistently maxed out while you still have send threads
available, you should increase the Max Send Jobs number.