Traditional methods of doing this on SA wiki talk about shell scripts as content filters.
Well that's an additional fork syscall on every email and the volume that will flow through the machine is significant. Not to mention that shell/bash scripts themselves are hardly efficient.
Is there a more efficient way of spam filtering in Postfix?
(and no, Amavis is not an option - that's an explicit requirement handed over to me).
Forking new processes is very low cost, this is what Unix/Linux is designed to do. Building a new spamassassin process for every email can be fairly expensive as it is a heavy weight process. For this reason spamd was created. It will fork off a few processes and keep them around to filter your email. The pool size is dynamic and tunable.
I prefer Exim4 which can run spamassassin in a couple of ways. These instructions indicate that you can configure the spassassassin using spamc (the spamd client) as a filter.
Looks like you need to read about spamd. Daemonized, can talk through sockets or ports.
