lammert - 3:43 pm on Feb 12, 2008 (gmt 0)

This will work to lower the load on your server during peaks, but it may also delay genuine emails. If you have only short bursts of spam coming in, this is the right solution, but if your general server load stays at 6 for an extended period of time, it may negatively affect your normal email stream because also normal email connections will be refused.

Your main problem is using SpamAssassin after the emails have been loaded. this causes load on sendmail, and on SpamAssassin to scan every mail.

If you already have SpamAssassin configured to automatically block emails from IP addresses which are listed in one of the remote blocklists (spamhaus, etc), you might consider putting that IP check not after sendmail, but before. In this way the email is rejected as soon as the spammer connects to your server and it doesn't consume processing power from sendmail and spamassassin.

Maybe blocking based on remote block lists (RBL) is a feature in sendmail, but to be honest I don't know; I am a qmail user. With my qmail installation I had to use an external small utility called rblsmtpd which fakes to be the real SMTP server until it has checked if the IP address is from a SPAM source. If the IP address is accepted, the SMTP connection is transparently rerouted to my real qmail SMTP server, otherwise the connection is dropped with an error message.