libmilter is a sendmail library for mail scanning by external programs. You can check to see whether your sendmail supports it with:
sendmail -d0 < /dev/null | grep MILTER
Sendmail 8.13 hase MILTER enabled by default.
With libmilter, sendmail can call other programs to help determine the disposition of a message. There can be multiple external programs, and sendmail makes calls at different points: when a sending machine connects, when it sends its HELO greeting, when it sends MAILFROM:, etc.
The milter itself can be written in any language: there are Perl milters, Python milters and more. The milter itself tells sendmail what parts of the process it wants to be involved in. It can accept (which means sendmail won't contact it again for that particular message), reject in various ways (which causes sendmail to reject or discard the message right then), or simply delay its decision until later in the process (which means that sendmail will send it more information that it has requested to be notified of). Multiple milters can process the same information; sendmail will call each unless and until one tells it to reject the message.
Everything you need to know about writing milters is here. I was a little amused that the authors gave advice on attracting spam in order to test a milter setup: I don't think too many of us need to worry about not having enough spam to test with.
Milters can be written in any language, but do require C hooks to interface with sendmail. As this book shows C code, you'll need at least read-only capability with C to get a lot out of it. However, it is also an excellent overview of spam in general, the techniques used and proposed for blocking, and how the spammers get around our efforts. I found that section of the book very interesting and learned a bit, too. There's fairly deep coverage here; even covering details like decoding mime.
If you are a C programmer (or can at least read it well enough to translate the concepts to other languages) interested in adding milters to sendmail, this is necessary reading.
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