I’m apparently old-school enough to find gmail and co painfully inefficient for handling significant amounts of mail. I still find procmail+mutt hard to beat. One thing mutt can’t do, however, is filter threads automatically - there’s no “mute” facility like gmail has; threads have to processed manually.

Equally, procmail itself has no threading facilities or understanding of Message-Id or References.

Matching email threads

It can be done, though, with some cheesy awk:

#!/bin/bash

#
# If a mail message has a References: value found in the refs file, then
# add the requested header.
#
# Usage:
#
# cat mail_msgs | match-thread.sh ~/.mail.refs.muted "Muted: true"
#

ref_file="$1"
header="$2"

mail=/tmp/match-thread.mail.$$
cat - >$mail

newrefs="$(cat $mail | formail -x references -x message-id | tr -d '\n')"

touch $ref_file

cat $ref_file | awk -v newrefs="$newrefs" '

	BEGIN {
		found = 0;
		split(newrefs, tmp);
		for (i in tmp) {
			refs[tmp[i]]++;
		}
	}

	# Each thread will have one line in the ref file, with
	# space-separated references. So we just need to look for any
	# reference from the mail.
	{
		for (ref in refs) {
			if (index($0, ref) != 0) {
				found = 1;
				exit(0);
			}
		}
	}

	END {
		exit(found ? 0 : 1);
	}
'

if [[ $? = 0 ]]; then
	cat $mail | formail -i "$header"
else
	cat $mail
fi

rm $mail

Essentially, we record all the References in the thread we’re trying to act on. Then we can trigger the above to see if the new mail is part of the thread of interest.

(This seems like the sort of thing formail could do, given its -D option has a message ID cache, but I haven’t even bothered to take a look at how hard that would be…)

procmail usage

In .procmailrc, we’d use this like so:

:0 Wfh: formail.lock
| $HOME/src/procmail-thread/match-thread.sh $HOME/.refs.muted "Procmail-Muted: true"

:0 Wfh: formail.lock
| $HOME/src/procmail-thread/match-thread.sh $HOME/.refs.watched "Procmail-Watched: true"

This will add the given header if we find any of the email’s References values in our “database”.

Then, we can do what we like with the mails, like deliver them as already-read, carbon copy them to the inbox, etc.:

:0
* Procmail-Muted: true
{
        SWITCHRC=$HOME/.procmailrc.markread
}

:0
* Procmail-Watched: true
{
        :0 c:
        $DEFAULT

        SWITCHRC=$HOME/.procmailrc.markread
}

:0
$DEST/

mutt usage

To actually watch or mute a thread, we add a couple of mutt macros:

macro index,pager "M" "|~/src/procmail-thread/add-thread.sh ~/.refs.muted<return>"
macro index,pager "W" "|~/src/procmail-thread/add-thread.sh ~/.refs.watched<return>"

The add-thread.sh script is similar to the above, but populates the refs file with all message IDs found in the given email.

I put all this in a git repo.