The instructions suggest "-o disable_dns_lookups" for the post
content_filter smtpd. This is not quite right as "disable_dns_lookups" is
used by the "smtp" client, not the "smtpd" server.

To get high throughput via a content_filter that is low_latency (runs
locally) but low concurrency (uses lots of CPU) one needs to avoid DNS
lookups in the sending "smtp" (content filter transport). Without this
setting the sending "smtp" substantially compounds the latency and
decreases throughput (for outbound mail whose destination domains may take
time to resolve). [This is not needed for low volume sites where average
content_filter concurrency is low.]

Since "-o disable_dns_lookups" is a *must* for the final "smtp" delivery
(not into the content_filter), the performance tweak requires to instances
of "smtp" in the master.cf with the "-o disable_dns_lookups" applied only
to the instance used by the content_filter.

For example:

        /etc/postfix/master.cf:
        smtp unix - - n - - smtp
        scan unix - - n - - smtp
                -o disable_dns_lookups=yes

        /etc/postfix/main.cf
        content_filter = scan:[127.0.0.1]:10025

This also allows one to defer the "scan" transport in an emergency
(until signature updates are available) without delaying delivery of
delay notices, bounces and already scanned deferred mail.

        # Uncomment to wait for signature updates:
        #defer_transports = scan

-- 
	Viktor.
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