Confusion between Reply To and Reply List in email responses

Definitely a Thunderbird thing as we discovered and tested around mid January:

Reply - one message - goes to [](

Reply-All - two messages - goes to both [](
and [](

Reply-to-List - one message - goes to

Turns out there is even an add-on for Thunderbird to fix this:


Yeah I strongly view this as a design error in Thunderbird.

However I sympathize with your users who are simply trying to do the right thing. I think about the best you can do at the moment is to customize the error message so that it is specific to your community and puts the most common error remedy at the top of the response, something like:

We’re sorry, but your email message to ["[project]"] (titled Re: [Subject]) didn’t work.

If you are using Thunderbird as your email client, you must reply to the list; it is not possible to reply directly to users via email.

Do you use more than one email address? Did you reply from a different email address? Email replies require that you use the same email address when replying. Alternately, the Message-ID header in the email may have been modified.

Not that exact text, of course – whatever you think makes sense, listed near the top.


I’ve updated our error message thanks to the tips in @simon’s message above and suspect that this will be sufficient to address the confusion that some in our community have felt. If there were a way to customize the “From:” address to something more like that would be icing on the cake, but for now I think we’re in a much better place.

Thanks again for all the help here,

PS — As long as we’re bagging on Thunderbird in this thread, I’ll mention that it’s still perplexing to our Thunderbird users that URLs from their HTML-formatted messages seem to get dropped on the floor by Discourse’s incoming mail processing filter, where it remains unclear to me whether this indicates a Thunderbird bug or a Discourse bug.


Based on this feedback, we’re also going to improve the error message in general. It will look like this in current beta and future releases:

We're sorry, but your email message to %{destination} (titled %{former_title}) didn't work.

Here are some things to check:

  - Do you use more than one email address? Did you reply from a different email address than the one you originally used? Email replies require that you use the same email address when replying.

  - Did your email software properly use the Reply-To: email address when replying? Unfortunately, some email software incorrectly sends replies to the From: address, which won’t work.

  - Was the Message-ID header in the email modified? The Message-ID must be consistent and unchanged.

Need more help? Reach out to us via the Contact Us details at  %{base_url}/about

But I can open my old discourse messages in Thunderbird without this problem. Only newer ones has this problem. I’ve described it in my post:


I guess you’re right and the presence of the List-Post header understandably makes Thunderbird think it’s a mailing list email. Probably a Reply-To header would suffice.

The following is from RFC 2369 - The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields

3.4. List-Post

The List-Post field describes the method for posting to the list. This is typically the address of the list, but MAY be a moderator, or potentially some other form of submission. For the special case of a list that does not allow posting (e.g., an announcements list), the List-Post field may contain the special value “NO”.


List-Post: (Postings are Moderated)
List-Post: NO (posting not allowed on this list)

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We don’t expose email addresses as that would be a privacy violation.

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I’m not an email expert so maybe wasn’t clear.

It seems that the emails have and - I’m only basing this on this topic, not from having checked, so could well be wrong.

I wondered whether and would work without confusing Thunderbird.

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The email notification headers we’re providing are correct. The mismatch comes from the user’s mental model of what is happening …

this user David is emailing me directly

… versus what is actually happening.

the forum is notifying me of activity on a topic

My hope is that the improved email error message will help teach the user the difference. It is a challenge to move email-only users to a hybrid web forum solution. :e-mail:


I understand that difference, but if you’re using Reply-To why also use List-Post?

I’m not saying it’s wrong - I just wonder. Purely based on this topic it seems to confuse Thunderbird and (if Reply-To works) to be unnecessary.

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IIRC, adding the header made some different mail software behave correctly. The history should be on Meta somewhere.


I’ve had a quick look. It seems that this is the genesis of the List-Post header: Emails are not threaded in Outlook 2013 - #5 by codinghorror

I’m not sure that it was fully thought through or that it was relevant to the problem being solved there (but again I’m still learning here myself).


Looks correct to me. What’s incorrect is mail software that ignores the Reply-To address … it’d be like you introducing yourself to me as Jonathan, but no, I insist on calling you Edward instead. :wink:


It sounds like Thunderbird uses the List-Post header for “Reply to List”, and the From header for “Reply” (ignoring Reply-To, which here is the same as List-Post). This makes some sense on some potential settings of a mailing list (e.g. the Reply-To might have been set by the mailing list rather than the original sender) as it would allow “Reply to List” to go to the list and “Reply” to go to the original sender (where otherwise it would also go to the list).

The absence of List-Post should remove the “Reply to List” button (Thunderbird 3 for users - Mozilla | MDN).

I guess that without the “Reply to List” option, the “Reply” button would send email to the address in the Reply-To header as expected.

I’ve got two related questions:

  1. Why does Discourse use the List-Post header (as you only ever want an email to go to the Reply-To address)?
  2. Does Thunderbird respect Reply-To when List-Post is not set?
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Good question! In my installation I didn’t check the mailing list feature, but now all post where send as mailinglist (with List-Post Header). In my opinion is this a bug.

Yes, before this change ( mail doesn’t have List-Post Header and I could answer to disourse with reply-to function.

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Unfortunately, that extension no longer works in the latest version of Mozilla Thunderbird. Is there a more up-to-date version of that extension?

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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.


Thank you for trying to get their attention! I hope that other people get involved too. :smile:


Good news! I finally received a response!