Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Roundcube - Web-based IMAP email client
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
HEY - Email at its best, new from Basecamp.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.