Nobody stops you from sending text/markdown MIME type. It wouldn't even be hard to render both HTML and plaintext from markdown. "Just do it" The hard part is

Nobody stops you from sending text/markdown MIME type. It wouldn't even be hard to render both HTML and plaintext from markdown. "Just do it" The hard part is to fix anything in most common e-mail clients, let alone add something new. One example being Thunderbird - notoriously stale tickets, undocumented codebase and nearly impossible to get anything merged.