mailcow - An open source mailserver suite.
Roundcube - Web-based IMAP email client
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
HEY - Email at its best, new from Basecamp.
Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.