If you’d like a simple, front-end spam filter that you can place in front of an existing SMTP mail server, try an OpenBSD transparent bridge running spamd. Here’s the no-nonsense version on undeadly.org, written by Chris Kuethe.
I did the same thing with a couple of Soekris 4801’s, running off 1GB Compact Flash cards. My 4801 model has a 266Mhz processor, which sustains around 10-15%% utilization during peak use: mass DNS lookups against the sbl-xbl.spamhaus.org list. 256MB RAM — 128 reserved as an MFS drive [RAM drive] for /var, using Bill Maas’ mfsmount script, 10MB reserved for dnscache, and the rest for general OS duty. No moving parts. Low power. Low heat.
I was perplexed at how all the spamd parts worked together, until I drew it out. Ah, now it makes sense. Hopefully you’ll find it helpful, as well.
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