Kiwi for Gmail - Dedicated Gmail client with clean integration of Google Calendar, Contacts, Google Drive, and...
mailcow - An open source mailserver suite.
Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
POP Peeper - POP Peeper is an email notifier that runs in your Windows task bar and alerts you when you have new...
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.