Email Threading for Professionals

Continuing my (likely unending) series of posts on email I want to talk about my latest in a Sisyphean line of tools to make the world suit my preferences.

As mentioned before I am a mutt user. Mutt, being not-Gmail, acts differently than what people have come to expect in 2016, though normally I can ignore other people’s expectations and move on. But I finally had to act in this case: email from the issue tracker we use at ZipRecruiter was not threading properly.

Threads are such a basic feature in email now that it’s almost hard to imagine what it would be like if they were lacking. At this moment my inbox has about fifty threads in it. If the email from my issue tracker were not threaded, I would have more than eighty, with very little context to help me read from one message to another. It is unconscionable that a bug tracker would send email and not thread right.

There are a couple possible theories as to why software might do this. The reason that I personally prefer is that Fog Creek is incompetent, but the more likely reason is that Gmail hides the problem…

Email Threading

Email threading is typically handled with three headers:

  • In-Reply-To
  • References
  • Subject

In-Reply-To is simply the unique Message-ID that the current email is (surprise!) a reply to. If you had all of the emails available to you, this would be sufficient. References solves the problem where one or more messages in the tree are missing, by storing a list of all Message-IDs from the parent to the root. Finally, as a last ditch effort the Subject header is used for to create threads from messages with subjects like Cleaning the kitchen and Re: Cleaning the kitchen. In Mutt these threads are called pseudo threads.

(Read more than you care to here.)

Gmail is problematic because it threads solely on the subject, and unlike other sloppy threading methods, the Re: is not required. This is why when people write emails with Subjects of Tomorrow they get pointlessly threaded together.

This implies that some software can simply set a stable subject and assume that users will get properly threaded email. I had not noticed this till recently because I simply filtered all of the email sent by FogBugz to trash. I have lately gotten better at handling my fulminating inbox and decided to make my life harder and turn on the firehose of issues.


When I first realized what was happening I had a few ideas on how to fix the threading, but finally ended up with: use the gmail API to both find unthreaded messages, find their full threads, and reupload them with the correct headers set. I would guess the total time spent for this took me about 4 to 8 hours. The complete code is long enough that I don’t think it makes sense to share in a blog post in it’s entirety so I’ll just link to it.

As a side note: the other way to do this would have been to use IMAP directly. The problem with that is that I also needed to use Gmail’s search to reconstruct the threads, and as far as I know the Gmail ids and the IMAP ids cannot be correlated without a bunch of silly work.

Oh and another side note: I could have patched Mutt to thread like Gmail does, but I’d also have to patch notmuch and I suspect changing how threading works would require a full index rebuild and it just seemed like a lot of effort to make working software act unnaturally.

I do want to share a few interesting bits though.

OAuth 2.0

I had so far not needed to learn how to authenticate with an OAuth service. For better or worse, I now know vaguely how OAuth works.

In my case the auth was split into three phases:

  1. Prompt the user to get the authentication code
  2. Get the authentication token with the code
  3. Maybe refresh the token with the refresh token if the auth token is too old.

My code for these looks something like this:

my $ua = LWP::UserAgent->new( keep_alive => 3 );

my $config = try {
} catch { +{} };

$config = get_token(
   refresh_token => $config->{refresh_token},
   grant_type => 'refresh_token',
) if $config->{refresh_at} && time > $config->{refresh_at};

sub get_token (%args) {
   my $res = $ua->request(POST '' => [
      client_id     => $gmail_api_client_id,
      client_secret => $gmail_secret,

   my $config_to_save = decode_json($res->decoded_content);

   my $config = {
      access_token => $config_to_save->{access_token},
      refresh_token => $config_to_save->{refresh_token} || $config->{refresh_token},
      refresh_at => time + $config_to_save->{expires_in},

   io ->file("$ENV{HOME}/.gmail-auth-token.json")


sub authenticate {
   my $uri = URI->new('');
      response_type => 'code',
      client_id => $gmail_api_client_id,
      redirect_uri => '',
      access_type => 'offline',
      prompt => 'consent',
      scope => '',
      include_granted_scope => 'false',

   print "Opening $uri ...\n";
   system 'xdg-open', "$uri";

   my $app = sub {
      my $env = shift;
      $env->{'psgix.harakiri.commit'} = 1;

      require Plack::Request;
      my $req = Plack::Request->new( $env );
      if (my $code = $req->param('code')) {
            code => $code,
            grant_type    => 'authorization_code',
            redirect_uri  => '',
         return [
            [ content_type => 'text/html' ],
            [ 'Success!' ]
      } else {
         return [
            [ content_type => 'text/html' ],
            [ sprintf "Error: %s (%s)", $req->param('error'), $req->param('error_subtype') ]

   require Plack::Runner;
   my $runner = Plack::Runner->new;
   $runner->parse_options(qw( --listen ));

That’s far from brief, but at the very least if someone else has the same problem I had it can serve as an example.


Here’s the bulk of the code that actually rethreads my email. It’s got enough little interesting bits that I added comments inline.

sub _rethread_emails ($fix_type, @email_ids) {
   my @emails;
   for my $id (@email_ids) {
      my $message = _do_req(
         "$id" => (
            format => 'raw',
      # WOW, it's not Base64, it's a weird alternate version of Base64 that's URI safe.
      # Thanks to haarg aka Graham Knop for helping with this.
      my $email = Email::MIME->new(urlsafe_b64decode($message->{raw}));
      push @emails, {
         email => $email,
         email_str => $email->as_string,
         id => $id,
         labelIds => $message->{labelIds},

   # FogBugz often sends multiple emails in the same thread at the exact same
   # time, so I added the Message-ID to the sort to stabilize it
   @emails = sort {
      find_date($a->{email_str}) <=> find_date($b->{email_str}) ||
      $a->{email}->header_raw('Message-ID') cmp $b->{email}->header_raw('Message-ID')
   } @emails;

   my $first_email = shift @emails;
   # We modify the Message-ID here, but prefixing or incrementing a prefix
   # *only* if the import headers (In-Reply-To and References) changed.  Same
   # as below.  Very important.
   prefix_message_id($first_email->{email}, $fix_type)
      if clear_initial_headers($first_email->{email});

   my $prev = $first_email;
   for my $email (@emails) {
      prefix_message_id($email->{email}, $fix_type)
         if update_references($email->{email}, $prev->{email});

      $prev = $email

   for my $email ($first_email, @emails) {
      my $new_raw = $email->{email}->as_string;
      # This is an elegant way to ensure that we don't upload if the email
      # doesn't change.
      next if $new_raw eq $email->{email_str};

      warn "Uploading replacement $email->{id}\n";
      # If you leave off the old labelIds you'll end up putting the email in
      # your archive, marked as read.
      my $upload_res = upload_email({ labelIds => $email->{labelIds}}, $new_raw);
      if (!$upload_res->is_success) {
         warn "Failed to upload $email->{id}: " . $upload_res->decoded_content . "\n";
      } else {
         if (decode_json($upload_res->decoded_content)->{id} ne $email->{id}) {
            warn "Deleting old $email->{id}\n";
            my $delete_res = delete_email($email->{id});
            if (!$delete_res->is_success) {
               warn "failed to delete $email->{id}: " . $delete_res->decoded_content . "\n";
         } else {
            # I found out at some point that if the Message-ID does not change,
            # when you upload an email and then delete the old version, you end
            # up just deleting what you uploaded.  I assume that the id from
            # gmail is a hash of the Message-ID or something.
            warn "id of $email->{id} did not change, something is probably wrong\n"

Multipart HTTP Uploads

I’ve never done a multipart upload in Perl before! It wasn’t too hard:

sub upload_email ($metadata, $email) {
   my $uri = URI->new('');
      uploadType => 'multipart',
      neverMarkSpam => 'true',
   my $req = HTTP::Request->new(
      POST => "$uri",
         Authorization => "Bearer $config->{access_token}",
         content_type => "multipart/related",
         HTTP::Headers->new(content_type => "application/json; charset=UTF-8"),
         [ content_type => "message/rfc822" ],
   $req->header( content_length => length $req->content );

So that’s about it! It sucks that I had to do all of this work, but it wasn’t too bad, and it’s been really reliable. Now that I have the core algorithm in place I keep seeing all kinds of messages that should be threading and aren’t. If you look at the code linked at the beginning of the posts you’ll see that I indeed am also rethreading AWS support emails. It’s nice to have this capability, though it would be nicer if these companies fixed their code.

Posted Wed, Nov 2, 2016