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Filtering email with filter(1)

Some time ago I was browsing through list of orphaned Debian packages^1. and encountered `filter` program with very promising description — "filter is one of the original mail filtering programs written for UNIX".

It is much simplier than `procmail`, its manual page is *page*, not *manual*, while supporting almost anything I need:

regex/substring matching of *Subject*, *To* and *From* headers

built-in delivery into mboxes and any other develiery method via piping. It is important to me, since I use mmh^2 mail user agent, which uses its own mail storage format.

support of filtering, based on length of message body. Not sure how well it copes with damned "content/alternative" messages.

Syntax of `filter` rules is much less cryptic and less verbose, than one of `procmail`. Compare

if (subject = "Foo") then save "/tmp/foo.mbox"

with much more verbose `procmail` rule.

:0:

	^Subject:.*Foo


/tmp/foo.mbox

Unfortunately, `filter` does not provide equivalence of two features of `procmail`, that are significant to my email workflow:

Matching of arbitrary header. My email provider run SpamAssassin on

incoming emails and adds *X-Spam-Level* header. My `procmail` configuration uses this header to classify messages as spam. Probably, I could run SpamAssassin locally, but why bother, if settings of my email provider are fine for me.

Handling of duplicate email. In cases when people reply to me and add mailing list into carbon copy, I receive two copies of email with same `Message-Id` header. It is distracting.

Both `filter` and `procmail` are long ~dead~ mature. Source code of `filter` is twice as short, if you care about such things. Half of that is regular expression engine, that is built it modern implementations of standard C library. Give `filter` a shot.

Definitely, `filter` would benefit from some care. Use standard regular expression engine, add support for *X-Spam-Level* header and here it is -- perfect email filtering solution. Maybe, one day, in my copious free time…