I don’t know. I don’t use Thunderbird and only came across it as I was researching the issue.
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.
Thank you for trying to get their attention! I hope that other people get involved too.
Good news! I finally received a response!
Oh fantastic! I hope they really did change it recently! Thank you for following up.
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.)
Well, there’s only one way to find out!
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.
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?
It isn’t a bug? It was a change that was implemented on purpose?
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.
Filed a but for this: Reply-key doesn't get expanded in email custom headers
What would you suggest we do to solve the initial problem instead of using
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.
Looks like we added the
List-Post header a while ago
Might not be necessary anymore. We’ll investigate.
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!
If it helps: Adding a Mail-Reply-To header with the identical email address as Reply-To would also fix the Thunderbird-Problem.
If removing the header helps, we should do that. Audit the code to make sure it’s not being used anywhere first though.