Debacle<p><span class="h-card" translate="no"><a href="https://babka.social/@kolev" class="u-url mention" rel="nofollow noopener" target="_blank">@<span>kolev</span></a></span></p><p>That depends on your mail client (mail user agent, MUA). Most clients allow adding custom header fields. Some also allow displaying arbitrary custom header fields.</p><p>It works e.g. with <a href="https://framapiaf.org/tags/aerc" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>aerc</span></a>, <a href="https://framapiaf.org/tags/claws" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>claws</span></a>, <a href="https://framapiaf.org/tags/mu4e" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>mu4e</span></a>, <a href="https://framapiaf.org/tags/KMail" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>KMail</span></a>, <a href="https://framapiaf.org/tags/MailApp" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>MailApp</span></a>, <a href="https://framapiaf.org/tags/mutt" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>mutt</span></a>, <a href="https://framapiaf.org/tags/Thunderbird" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>Thunderbird</span></a>…</p><p><a href="https://wiki.xmpp.org/web/Jabber_Email_Header" rel="nofollow noopener" translate="no" target="_blank"><span class="invisible">https://</span><span class="ellipsis">wiki.xmpp.org/web/Jabber_Email</span><span class="invisible">_Header</span></a></p><p><a href="https://framapiaf.org/tags/Jabber" class="mention hashtag" rel="nofollow noopener" target="_blank">#<span>Jabber</span></a></p>