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Is there a way with procmail (or something else?) to search for email where the from: and to: fields both contain the same address? And can it be done without needing to hardcode each and every email address in a recipe?

Basically I'm trying to find emails where the sender sends the mail to their own address then bcc's everyone else. Normally I use notmuch and I'm happy with that but it can't do what I'm trying to achieve here. notmuch can work together with procmail though so I was hoping someone who knows procmail really well could help and thus save me from needing to learn a whole new filtering system to do this one thing.

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  • should you not search into logs rather than procmail? Oct 28 '16 at 19:26
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This is possible, though it requires some rather obscure Procmail features, and of course, you need to understand what you are achieving.

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*   ^From:[     ]*\/[^  ].*
* $ ^To:[   ]*$\MATCH
{ ... actions ... }

The \/ capture operator collects the matching string into the special variable MATCH. On the next line, we search for the same string in the To: header. Where $MATCH contains the captured string, $\MATCH contains a regex where any regex special characters in the string have been escaped so as to match literally. The $ modifier is required on the recipe in order to allow interpolation of Procmail variables into the regex.

This uses the usual [ ]* (space or tab, zero or more) to skip over whitespace after the colon, then grabs everything starting from the first non-whitespace character.

In practice, this means that the From: and To: headers have to have identical contents. If the sender puts in a different "real name" (which technically is just a comment), this recipe will fail to match. For example,

From: Myself <me@example.net>
To: Everyone <me@example.net>

Here, the email terminus is identical, but the whole field is different. If you need to cope with this scenario, maybe change the regex to match up through the first < before the \/ but this will obviously depend on the precise strings you need to match; there are many possible variations.

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