Thunderbird appears to be the only email reader that gets close to supporting mid:// urls. https://superuser.com/questions/681421/what-email-clients-support-ci

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