Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Postfix Admin - Postfix Admin is a web based interface used to manage mailboxes, virtual domains and aliases.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
mailcow - An open source mailserver suite.