Mailspring is a multi-platform email client.

EspoCRM is an Open Source CRM (Customer Relationship Management) software that allows you to see, enter and evaluate all your company relationships.

ActiveInbox for Gmail is an email management software for Gmail.

Nylas Mail is a open source, extensible email app.

Kopano is an open-source groupware application suite originally based on Zarafa.

mailcow is a mail server suite.