I don’t know. I don’t use Thunderbird and only came across it as I was researching the issue.
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I’m attempting to get into contact with the Thunderbird team. They should be able to offer further assistance and determine if this really is an issue with their software or not.
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Thank you for trying to get their attention! I hope that other people get involved too. 
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Good news! I finally received a response!
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Oh fantastic! I hope they really did change it recently! Thank you for following up.
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Would an alternative be for Discourse to stop using List-Post (and continue just to use Reply-To)?
(I think List-Post was added blindly/irrelevantly while solving another problem and caused this problem.)
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Well, there’s only one way to find out! 
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I ran into this problem and after some digging found that the culprit for Thunderbird’s behaviour is the ‘mail.override_list_reply_to’ configuration variable. At least in recent versions of TB it ships with a default boolean value of True. Setting this variable to False allows a simple reply to respect the value Discourse sets in the ‘Reply-to’ header. You can test this by accessing the Config editor from the Thunderbird preferences menu and searching for the ‘mail.override_list_reply_to’ variable and toggling the value False.
I don’t think it is practical to ask all users to change their Thunderbird configuration, but referring to this variable might be helpful in talks with Mozilla about this issue.
Update: seems like TB actually introduced this since release 52.4.0
And this is the bug report where someone decided this was a good idea.
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Yes, it seems to be a combination of that and Discourse’s unnecessary use of the List-Post header.
Was also struggling with this behavior and found out, that they might actually have put some thinking into it, when implementing it in Thunderbird: Thunderbird:Help Documentation:Mail-Followup-To and Mail-Reply-To - MozillaWiki
- If you reply-to-author and Mail-Reply-To is set in the original message, Thunderbird uses it instead of Reply-To or From. This works since Thunderbird-1.5.
The difference I found explained at https://www.gnu.org/software/emacs/manual/html_node/emacs/Mail-Headers.html :
‘Mail-Reply-To’: This field takes precedence over ‘Reply-To’. It is used because some mailing lists set the ‘Reply-To’ field for their own purposes (a somewhat controversial practice).
Now I tried injecting this header via “email custom headers”, but unfortunately reply_key does not get expanded properly. Am I doing something wrong?
Mail-Reply-To: sender-name+%{reply_key}@domain.net
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It isn’t a bug? It was a change that was implemented on purpose? 
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The underlying problem is that Discourse unnecessarily sets the List-Post header, which makes Thunderbird think it’s a mailing list, which affects the replying options presented by Thunderbird.
Discourse started to use List-Post almost by accident. It was added when fixing another problem, but wasn’t necessary to solve that other problem, and at the same time caused this problem.
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What would you suggest we do to solve the initial problem instead of using List-Post headers?
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It’s a while since I looked at it, and I might have been wrong, but I think two or three headers were all added at once to solve that problem, whatever it was, and List-Post just wasn’t necessary.
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Looks like we added the List-Post header a while ago
Might not be necessary anymore. We’ll investigate.
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I wrote a few posts about it here:
Here is the original problem described:
List-Post was blindly copied from somewhere else without any consideration being given to what it is for. It hasn’t anything to do with threading - as far as I know anyway! 
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If it helps: Adding a Mail-Reply-To header with the identical email address as Reply-To would also fix the Thunderbird-Problem.
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If removing the header helps, we should do that. Audit the code to make sure it’s not being used anywhere first though.
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