Roundcube - Web-based IMAP email client
mailcow - An open source mailserver suite.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.