<p>I tested this change  on master.apis.dev now. Results look good in Thunderbird: comments to a note from different people, close, reactive, are all in one thread, in they share the same In-Reply-To header.</p>
<p>However, Google Mail now starts merging emails that share the same subject, although the HTTP References header differs. Even worse, different types of messages are now grouped, even they belong to different objects.</p>
<p>I think one way of addressing this would be to use less generic email title (see my comment here: <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="520895867" data-permission-text="Issue title is private" data-url="https://github.com/openstreetmap/openstreetmap-website/issues/2430" data-hovercard-type="issue" data-hovercard-url="/openstreetmap/openstreetmap-website/issues/2430/hovercard?comment_id=552435433&comment_type=issue_comment" href="https://github.com/openstreetmap/openstreetmap-website/issues/2430#issuecomment-552435433">#2430 (comment)</a>).</p>

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