Procmail is a powerful Mail Delivery Agent (MDA) that can be
used for any number of things including personal mail filtering, mail delivery
on servers, and much more. More often than not, individuals using fetchmail to download
e-mail will use procmail to file and deliver it to its final destination—typically
an mbox file somewhere on the user’s hard drive.

Many users have GUI e-mail clients that can read standard mbox
files, but because of procmail’s flexibility, they opt to use fetchmail and
procmail to download and filter their mail rather than their e-mail client.

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For instance, procmail can filter messages based on pretty
much any criteria you for which you can write a regular expression. If you
wanted to filter mailing list messages into separate mbox files, you could do
so based on the Return-Path e-mail header like this:

* ^Return-Path:.*
* ^Return-Path:.*

This snippet of a ~/.procmailrc
file uses the regular expression denoted by the line beginning with an asterisk
(*); in this case, we are looking for a line in the e-mail message that begins
with Return-Path, has any number of
spaces or characters, and also contains the e-mail address we want to base the
filter on. The next line contains the path of the mbox to which the message delivers
if it matches.

The final line in a .procmailrc
might look like:


This is a fairly drastic measure, but essentially it means
that if none of the previous criteria matched, deliver the message to /dev/null, which would discard the
message entirely. A more likely scenario would be to change this to something
like /Users/joe/Mail/Inbox where all
unfiltered mail would go.

Procmail is so powerful that these expressions are called
recipes, and recipes have even been generated to discard spam. You can also
rewrite parts of messages based on the content. More information on procmail
can be found at, and you can
install it from your Linux vendor’s installation medium; procmail comes with
all Linux distributions.