MailerSend - Better transactional emails for teams.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
HEY - Email at its best, new from Basecamp.
Geary - Geary is an email application built for GNOME 3.
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.