Bojler
Bojler is an email framework for developing responsive and lightweight email templates that will render correctly across each of the most popular email clients.
10 Alternatives To Bojler
Afterlogic Webmail
Webmail is an open-source webmail script for existing IMAP servers.
MJML
The open source framework for responsive emails
Mailpile
Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Mailspring
Mailspring is a mail client for Mac, Windows and Linux.
Mu4e
Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Rainloop
RainLoop is a web based email client.
Roundcube
Web-based IMAP email client
SOGo
SOGo is groupware server with a focus on scalability and open standards.
Squirrelmail
SquirrelMail is a standards-based webmail package written in PHP.
Tutanota
Get your encrypted mailbox for free.