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qmail by John Levine

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Tuning Qmail

More often than not, qmail doesn't need any tuning. It's designed to work well on typical Unix systems. For local deliveries, qmail is usually disk-bound, because it syncs files and directories to disk to avoid losing mail if the system crashes. Although it's possible on some systems to set filesystem parameters to subvert the syncs, that's usually a poor economy. If you want your local mail delivered faster, get a faster disk.[2] If your system has a lot of unusually slow local delivery programs, or it runs really slow spam filters (Spamassassin can fall into that category), it's possible that local deliveries could be CPU-bound. The easiest way to find that out is with a utility like top that shows what's running. Much of the slowness in slow spam filters is due to DNSBL lookups, which are in fact network bound. Modern CPUs are so fast that it's a rare mail system that is even occasionally compute-bound.

Remote deliveries are invariably network-bound. If the goal is to deliver mail as fast as possible, crank the concurrency up as high as possible. Looking at the zoverview results, it completed deliveries of 309400658 to 65158 recipients, for an average of a little under 5 Kbps per message. The average xdelay was 5.8 seconds, so each delivery was sending under 1 Kbps. This system happens to be on a T1 line, which can transmit 192 Kbps (that's 1.5 megabits divided by 8 bits per byte). So if each delivery sends 1 Kbps and the channel is 192 Kbps, it takes about 192 simultaneous ...

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