[Mailman-Users] Reply filter

Mark Sapiro mark at msapiro.net
Tue Sep 27 23:33:36 CEST 2011

Richard Damon wrote:
>The question is, can a spam filter be set up to check both the From 
>fields and the To/CC fields?

Yes, but it's awkward at best. header_filter_rules are Python regular
expressions that are matched against the entire set of message headers
in MULTILINE and IGNORECASE mode. The awkwardness comes about because
you don't know if the From: precedes or follows the To: or Cc: so you
need to check both possibilities. A regexp like

^From:.*user1 at example\.com[^\177]*^(To:|Cc:).*user2 at example\.net'

will match a From: header containing user1 at example.com followed by a
To: or Cc: header containing user2 at example.net.

Note the use of [^\177]* to skip all the intervening headers between
From: and To: or Cc:. [^\177] will match anything that isn't a rubout
(octal 177, hex 7F) including newlines. There should be no rubouts in
email message headers. You can't use .* here because . won't match a
newline, and you can't use (?s) to set 'dot matches all' because that
would allow .*user1 at example\.com to match beyond the From: header.

Then you could add a second regexp to the rule

^(To:|Cc:).*user1 at example\.com[^\177]*^From:.*user2 at example\.net'

to cover the case where the To: or Cc: precedes From: and then add two
more with the roles of user1 and user2 reversed.

You might get away with combining all this into one as

^(From:|To:|Cc:).*(user1 at example\.com|user2 at example\.net)[^\177]*^(From:|To:|Cc:).*(user1 at example\.com|user2 at example\.net)'

(all on one line). This would match any From:, To: or Cc: containing
either user1 at example.com followed by a From:, To: or Cc: containing
user2 at example.net. The drawback of this all in one is it would match a
message From: a 3rd party To: one of the users with Cc: to the other
and would match a message from one of the users with a Cc: to herself.

Mark Sapiro <mark at msapiro.net>        The highway is for gamblers,
San Francisco Bay Area, California    better use your sense - B. Dylan

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