Postfix Admin - Postfix Admin is a web based interface used to manage mailboxes, virtual domains and aliases.
Jam.py Postfix Aliases - Web App to manage only Postfix Aliases with a LARGE number of managed Domains (>hundreds).
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
Virtualmin - It includes numerous features not in the GPL version (like script installers, resellers, HTML editor, an improved UI, mobile access, spam and virus filtering).
mailcow - An open source mailserver suite.
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.