Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Typora - A minimal Markdown reading & writing app.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.