Thunderbird appears to be the only email reader that gets close to supporting mid:&#x2F;&#x2F; urls.<p><a href="https:&#x2F;&#x2F;superuser.com&#x2F;questions&#x2F;681421&#x2F;what-email-clients-support-cid-and-mid-uris" rel="nofollow">https:&#x2F;&#x2F;superuser.com&#x2F;questions&#x2F;681421&#x2F;what-email-clients-su...</a><p>It uses the &quot;thunderlink:&quot; protocol to support this. It does require some OS-specific setup.<p><a href="https:&#x2F;&#x2F;addons.mozilla.org&#x2F;en-US&#x2F;thunderbird&#x2F;addon&#x2F;thunderlink&#x2F;" rel="nofollow">https:&#x2F;&#x2F;addons.mozilla.org&#x2F;en-US&#x2F;thunderbird&#x2F;addon&#x2F;thunderli...</a><p>Linking directly to email messages as transparently as linking to webpages is useful, and I wish it were more widespread.