Replying To Old Mailing List Threads

One common complains on the Swift Evolution mailing list is about its inscrutable interface. If you see an inactive thread on the web archive but haven't subscribed, there seems to be no way to participate or "revive" it since you never received any of its emails.

With a hint and some experiments, I've discovered that there is a way. This post reveals the magic step by step (without commenting on the merits of email or this solution).

Like HTTP, Email is a text-based protocol. Unlike HTTP, we directly consume payloads of this protocol and, as a consequence, don't think much about that very fact.

Like HTTP, each email includes headers hidden by normal email clients. Each header is essentially two strings, a key and a value, separated by a colon. Among these headers are Message-Id and In-Reply-To. When we tell our email client applications to start a reply, they take the value from Message-Id and use it as value for In-Reply-To automatically.

To observe all this in action, we can open some emails with our favorite text editors and look for these headers. Of course, this require us to know where the emails exist as files. On macOS, an easy thing to do is to drag from to Finder/Desktop and open the resulting .eml file:

Open an email in text editors

Among the (perhaps overwhelming amount of) headers, we'll find the two fields we are looking for:

Email Headers Message-Id and In-Reply-To

… I'll leave the clients' reply behavior regarding these fields for the reader to verify.

Mailing list software such as GNU Mailman, which and use to host various mailing lists, associate emails in threads by chaining them with the headers explained above, among other things. As long as we have a message's Message-Id, we can reply to it "officially" by adding its value to the In-Reply-To header in our email, regardless of whether we have received that email ourselves previously.

So here are the steps to reply retroactively to a mailing list thread.

  1. find Message-Id of the mailing list message we want to reply to.

    This value is contained in the mailing list web archive's mailto link. Unfortunately doesn't recognize it. It's easy enough to find it ourselves though:

    Finding Message-Id on mailman archive

    Note the <, >, @ characters are percent-quoted. We have to recover the id values to the format <[email protected]>.

    Another way to acquire this value is from the gzip'd archive. There they just exist as plain text. The downside is you have to dig out the message itself first.

  2. Add In-Reply-To header to our email.

    Draft a new email, make its title the same as our email chain's title (this is a good idea because lots of email clients do use the title to thread messages). Set the appropriate recipients, and CCs, including the mailing list's address. Now save this email as draft and open it in a text editor as we did in our investigations. Add in the line In-Reply-To: <>, save it. Then send this email (for example, open it with and use Message->Send Again in the menu).

    Of course, some email client supports adding it from their GUI.

You'll find the in addition to functioning as an normal message to the recipients, the mailing list will properly put your message to the original thread (in the case of swift-evolution, only if your reply is within the same week).