<p>First idea for an implementation would be something along those lines:</p>
<div class="highlight highlight-source-ruby"><pre>diff <span class="pl-k">--</span>git a<span class="pl-k">/</span>app<span class="pl-k">/</span>mailers<span class="pl-k">/</span>notifier.rb b<span class="pl-k">/</span>app<span class="pl-k">/</span>mailers<span class="pl-k">/</span>notifier.rb
index b12599981..98d4946d8 <span class="pl-c1">100644</span>
<span class="pl-k">---</span> a<span class="pl-k">/</span>app<span class="pl-k">/</span>mailers<span class="pl-k">/</span>notifier.rb
<span class="pl-k">+++</span> b<span class="pl-k">/</span>app<span class="pl-k">/</span>mailers<span class="pl-k">/</span>notifier.rb
@@ <span class="pl-k">-</span><span class="pl-c1">99</span>,<span class="pl-c1">6</span> <span class="pl-k">+</span><span class="pl-c1">99</span>,<span class="pl-c1">8</span> @@ <span class="pl-k">class</span> <span class="pl-c1">Notifier</span> <span class="pl-k"><</span> <span class="pl-c1">ApplicationMailer</span>
 
       attach_user_avatar(comment.user)
 
<span class="pl-k">+</span>      headers[<span class="pl-s"><span class="pl-pds">'</span>In-Reply-To<span class="pl-pds">'</span></span>] <span class="pl-k">=</span> <span class="pl-s"><span class="pl-pds">"</span>osm-diary-<span class="pl-pse">#{</span><span class="pl-s1">comment.diary_entry.id</span><span class="pl-pse">}</span>@<span class="pl-pse">#{</span><span class="pl-s1"><span class="pl-c1">Settings</span>.server_url</span><span class="pl-pse">}</span><span class="pl-pds">"</span></span>
<span class="pl-k">+</span>
       mail <span class="pl-c1">:from</span> => from_address(comment.user.display_name, <span class="pl-s"><span class="pl-pds">"</span>c<span class="pl-pds">"</span></span>, comment.id, comment.digest, recipient.id),
            <span class="pl-c1">:to</span> => recipient.email,
            <span class="pl-c1">:subject</span> => <span class="pl-c1">I18n</span>.t(<span class="pl-s"><span class="pl-pds">"</span>notifier.diary_comment_notification.subject<span class="pl-pds">"</span></span>, <span class="pl-c1">:user</span> => comment.user.display_name)
@@ <span class="pl-k">-</span><span class="pl-c1">136</span>,<span class="pl-c1">6</span> <span class="pl-k">+</span><span class="pl-c1">138</span>,<span class="pl-c1">8</span> @@ <span class="pl-k">class</span> <span class="pl-c1">Notifier</span> <span class="pl-k"><</span> <span class="pl-c1">ApplicationMailer</span>
       <span class="pl-smi">@author</span> <span class="pl-k">=</span> <span class="pl-smi">@commenter</span>
       attach_user_avatar(comment.author)
 
<span class="pl-k">+</span>      headers[<span class="pl-s"><span class="pl-pds">'</span>In-Reply-To<span class="pl-pds">'</span></span>] <span class="pl-k">=</span> <span class="pl-s"><span class="pl-pds">"</span>osm-note-<span class="pl-pse">#{</span><span class="pl-s1">comment.note.id</span><span class="pl-pse">}</span>@<span class="pl-pse">#{</span><span class="pl-s1"><span class="pl-c1">Settings</span>.server_url</span><span class="pl-pse">}</span><span class="pl-pds">"</span></span>
<span class="pl-k">+</span>
       subject <span class="pl-k">=</span> <span class="pl-k">if</span> <span class="pl-smi">@owner</span>
                   <span class="pl-c1">I18n</span>.t(<span class="pl-s"><span class="pl-pds">"</span>notifier.note_comment_notification.<span class="pl-pse">#{</span><span class="pl-s1"><span class="pl-smi">@event</span></span><span class="pl-pse">}</span>.subject_own<span class="pl-pds">"</span></span>, <span class="pl-c1">:commenter</span> => <span class="pl-smi">@commenter</span>)
                 <span class="pl-k">else</span>
@@ <span class="pl-k">-</span><span class="pl-c1">166</span>,<span class="pl-c1">6</span> <span class="pl-k">+</span><span class="pl-c1">170</span>,<span class="pl-c1">8</span> @@ <span class="pl-k">class</span> <span class="pl-c1">Notifier</span> <span class="pl-k"><</span> <span class="pl-c1">ApplicationMailer</span>
 
       attach_user_avatar(comment.author)
 
<span class="pl-k">+</span>      headers[<span class="pl-s"><span class="pl-pds">'</span>In-Reply-To<span class="pl-pds">'</span></span>] <span class="pl-k">=</span> <span class="pl-s"><span class="pl-pds">"</span>osm-changeset-<span class="pl-pse">#{</span><span class="pl-s1">comment.changeset.id</span><span class="pl-pse">}</span>@<span class="pl-pse">#{</span><span class="pl-s1"><span class="pl-c1">Settings</span>.server_url</span><span class="pl-pse">}</span><span class="pl-pds">"</span></span>
<span class="pl-k">+</span>
       mail <span class="pl-c1">:to</span> => recipient.email, <span class="pl-c1">:subject</span> => subject
     <span class="pl-k">end</span>
   <span class="pl-k">end</span>
</pre></div>

<p style="font-size:small;-webkit-text-size-adjust:none;color:#666;">—<br />You are receiving this because you are subscribed to this thread.<br />Reply to this email directly, <a href="https://github.com/openstreetmap/openstreetmap-website/issues/2430?email_source=notifications&email_token=AAK2OLIYS2XQ4BOXPHKJVCLQXQO3BA5CNFSM4JLUCXZ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEGGQYNI#issuecomment-562891829">view it on GitHub</a>, or <a href="https://github.com/notifications/unsubscribe-auth/AAK2OLMTC4X3UEQIAH6DLCLQXQO3BANCNFSM4JLUCXZQ">unsubscribe</a>.<img src="https://github.com/notifications/beacon/AAK2OLLU7GV5XBDJN7XYNPLQXQO3BA5CNFSM4JLUCXZ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEGGQYNI.gif" height="1" width="1" alt="" /></p>
<script type="application/ld+json">[
{
"@context": "http://schema.org",
"@type": "EmailMessage",
"potentialAction": {
"@type": "ViewAction",
"target": "https://github.com/openstreetmap/openstreetmap-website/issues/2430?email_source=notifications\u0026email_token=AAK2OLIYS2XQ4BOXPHKJVCLQXQO3BA5CNFSM4JLUCXZ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEGGQYNI#issuecomment-562891829",
"url": "https://github.com/openstreetmap/openstreetmap-website/issues/2430?email_source=notifications\u0026email_token=AAK2OLIYS2XQ4BOXPHKJVCLQXQO3BA5CNFSM4JLUCXZ2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEGGQYNI#issuecomment-562891829",
"name": "View Issue"
},
"description": "View this Issue on GitHub",
"publisher": {
"@type": "Organization",
"name": "GitHub",
"url": "https://github.com"
}
}
]</script>