Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Roundcube - Web-based IMAP email client
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.