6.5. MTA Performance Tuning Tips for Qmail
The 'big knob' for turning up the speed in Qmail is the setting in 'concurrencyremote', which typically lives in /var/qmail/control. The default value is something like 20, and you can turn it up as high as 120 in a default installation. If you need to go faster, there is a 'big concurrency' patch for Qmail that will allow it to be set higher. I have some mail servers that have it set to 250 for delivering email notifications for earthquakes. They typically manage between 1,000-1,500 deliveries per minute when running at full speed.
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