Postfix Admin - Postfix Admin is a web based interface used to manage mailboxes, virtual domains and aliases.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
ViMbAdmin - ViMbAdmin (pronounced vim-be-admin and meaning Virtual Mailbox Administration) is intended as an...
NeutronMail - Self-hosted server for the ProtonMail client.
Ratuus - RATUUS is an easy to use, web based system for administration of POSTFIX virtual domains and users
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).