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
https://github.com/discourse/discourse/commit/beaa1455729c8a442e37e9f23d908fa7677ce0cb
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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@nbianca investigated and removed it in this commit. She replied in
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