Splitting email into IMAP folders

26 Jun, 2004

After years of using Emacs-based Gnus to read my email, I've recently switched to IMAP, and Thunderbird.

Having switched, I immediately began to miss the powerful mail-splitting functionality of Gnus. I subscribe to a bunch of mailing lists, and had configured Gnus such that messages were filed into a separate folder for each list. For example:

(setq nmail-split-methods
      '(
        ("list.yahoo.\\1"       
         "^Delivered-To:.* \\(\\S +\\)@yahoogroups.com")
        ("list.codehaus.\\1" 
         "^Delivered-To:.* \\(\\S +\\)@codehaus.org")
        ("misc" "")))

I wanted to do the same kind of thing in my new IMAP-based environment. And I want to be able to read my mail from a variety of locations, which indicated a server-side solution, rather than one depending on filters in my mail client.

The result is a bit of magic using procmail, though procmail alone wasn't quite up to the job, so I had to resort to a bit of Ruby on the side. Here's the relevant part of my ~/.procmailrc ...

DEFAULT="$HOME/Maildir/"

:0 hWi
AUTOFOLDER=| $HOME/Global/bin/choose-imap-folder

:0
* ! AUTOFOLDER ?? ^^^^
${DEFAULT}.${AUTOFOLDER}/

The first recipe pipes the message header into a script, and captures the script output in $AUTOFOLDER. The next checks to see whether $AUTOFOLDER is set, and if it is, delivers to the named folder.

Paths above are correct for Courier IMAPD, where mail intended for the "foo" folder needs to be filed in "$HOME/Maildir/.foo/".

The folder-selection script itself looks something like this:

#! /usr/bin/env ruby

ARGF.each_line do |line|

  folder = \
  case line
    
  when %r{^Delivered-To: mailing list (\S+)@yahoogroups\.com}
    'yahoo.' + $1

  when %r{^Delivered-To: mailing list (\S+)@codehaus\.org}
    'codehaus.' + $1
  when %r{^Delivered-To: mailing list (\S+)@(\S+)\.codehaus\.org}
    'codehaus.' + $2 + '-' + $1
    
  end

  if folder 
    puts folder
    exit
  end

end

So there you go. I'll have to tweak the choose-imap-folder when I subscribe to a list from a new source, but otherwise, new folders just appear like magic.

To make this comfortable when using Thunderbird, I had to do two things: