How to manually set the "In-Reply-To" header in Thunderbird?

I wanted to join a mailing list thread on a list to which I subscribed after the thread started. I wanted my reply to follow an existing message in the thread.

I researched how I could manually set the In-Reply-To header, finding this which basically suggests:

it must be added to the list of custom headers by doing:

Edit → Preferences → Advanced → General → Config Editor

Search for mail.compose.other.header and add In-Reply-To

Then In-Reply-To will show up as one of the options in the compose window in the same place where you can choose To:, Cc:, etc.

Well I did that, and the field did appear (after restarting Thunderbird). I entered the message id in the field and sent the messagge. But the header, although present, was blank:

To: "he...
In-Reply-To: 
From: ....
Subject: Re: ...

Is it possible to manually set In-Reply-To in a Thunderbird message ?

I was also bit by this same problem. I have then tried the header-tools-lite extension that can be used to edit the header of email messages. I created a new message, saved it as draft, edited the header (References field) and finally sent the message. It seems to work fine.

As mirh hinted in the comments of Luca’s answer, it seems Thunderbird will throw out the In-Reply-To header value unless the message ID is surrounded in angle brackets.

So, instead of choosing In-Reply-To: as a header and entering the value [email protected], instead use the value <[email protected]>.

I tested this on a Mailman list, and after additionally setting CC: to the mailing list email, my message was threaded properly in the list.

comment 1189889/1098266 mentions using Header Tools Lite. This is an old XUL extension and no longer will work with Thunderbird 68+, developer says on their website:

On the spotlight - END OF DEVELOPMENT AND SUPPORT

As is well known, Mozilla has chosen to renounce XUL/XPCOM based technology for extensions and has decided to replace it with Webextensions.

For non-experts, this means that extensions now use a completely different technology and that all "old style" extensions must be rewritten from scratch to work.

Not having enough time and motivation for such a complicated job, as well as for personal reasons, I decided to completely end the development and support of all my extensions.

There is this alternative however, which is how I do it:

Borrowed from MozillaZine: Custom headers. Archive link 1, 2. Just in case it disappears.

1. Check the value of: mail.identity.idX.useremail to determine which index number refers to your email account. ie Edit → Preferences → Advanced → General → Config Editor
2. Set user_pref("mail.identity.id1.headers", "References, InReplyTo"); to create the custom headers.
3. Change the custom headers, you could use a user.js for this.
4. Set the values for References, you can use as many References as you like, just make sure they are separated with a new-line character and a space ie: \n. user_pref("mail.identity.id1.header.References", "References: <[email protected]>\n <[email protected]>");
5. Set the In-Reply-To: ie user_pref("mail.identity.id1.header.InReplyTo", "In-Reply-To: <[email protected]>");
6. Restart Thunderbird and send your email.
7. Comment out the above set options in your user.js with /* */. Go back into about:config ie (Edit → Preferences → Advanced → General → Config Editor) and rightclick on the keys mail.identity.id1.headers, mail.identity.id1.header.References and mail.identity.id1.header.InReplyTo and click Reset. Do not set these values to anything else in a user.js, because it will interfere with Thunderbird's normal behavior when you click Reply on an email.