Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
sSMTP - sSMTP is a simple MTA to deliver mail from a computer to a mail server.
Roundcube - Web-based IMAP email client
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.