The best open source alternative to Outlook.com is Proton Mail. If that doesn't suit you, our users have ranked more than 50 alternatives to Outlook.com and 16 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Outlook.com are Tuta Mail, Disroot, Posteo and Infomaniak kMail.
Columba is an email client written in Java, featuring a user-friendly graphical interface with wizards and internalionalization support. Its a powerful mail management tool with features to enhance your productivity and communication.
Fripost – the Free Email Association. Fripost organize the people who want an alternative to "free" e-mail services where you are constantly under surveillance and personal data is handed over to third parties.
Unofficial, free and open-source Google Inbox Desktop App. Built using electron, Inboxer includes a menu bar which is globalmenu compatible and works, and keyboard shortcuts.
