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Howto late-join a mail thread seamlessly technically

Say you joined a mailing list just to jump onto a specific mail thread that someone brought to your attention. To have your mail join the thread seamlessly (in a way many mail clients support) you need

  1. the mail's subject
  2. the message id of the parent to attach to
  3. a mail client allowing to add headers In-Reply-To and References.

The message ID header is easy to find once you have mail headers at hand:

Message-ID: <>

If you#re lucky you get them from a mail archive or a friend who's subscribed already. Now a child of that mail would carry these headers:

In-Reply-To: <>
References: <>

If you're using Thunderbird, you can make it show headers In-Reply-To and References for selection in the mail composition window. All you need to do is open the advanced config GUI editor (sometimes called about:config, Menu > Edit > Preferences > Advanced > Config Editor ...), select field mail.compose.other.header and set it to In-Reply-To,References. I learned about that field myself from here. If you have updates on this procedure, please comment below. Thanks!