Well,
Neither I nor Speakeasy.net has been able to come up with a good answer regarding the frequent "Connection timed out" log messages being reported by couriersmtpd.
"Connection timed out" is reported by courieresmtpd when the connecting client does not send any commands for ten minutes.
If you are logging this only for certain sending IP address ranges, it is more likely to be a sender-specific problem.
If you are logging this for random connections, and you are using a firewall, the firewall is probably broken: it uses a much shorter inactivity timeout, and drops the connection after a too-short inactivity period, without bothering to reset either side of the socket, which still thinks it's connected to its peer. The connecting client will find out as soon as it sends the next command. The server will wait until the inactivity timer expires.
I'm not using any blackhole lists directly through courer, although I do have spamassassin integrated in a system-wide maildrop filter.
Could delays in spamassassin processing (possibly due to a misconfiguration of spamassassin) be the cause of connection timeouts in couriersmtpd?
If you're invoking the filter in etc/maildroprc, then no.
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