SOGo

SOGo is groupware server with a focus on scalability and open standards.

Afterlogic Webmail

Webmail is an open-source webmail script for existing IMAP servers.

Google’s encompassing suite of cloud-based business apps.

Horde Groupware is a free, enterprise ready, browser based collaboration suite.

Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.

Sharing & communication software for business

Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.

Mailspring is a mail client for Mac, Windows and Linux.

Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.

Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.

Self-hosted server for the ProtonMail client.

Secure email with absolutely no compromises. Get your free encrypted email account today.

The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.

RainLoop is a web based email client.

Web-based IMAP email client

SquirrelMail is a standards-based webmail package written in PHP.

Get your encrypted mailbox for free.

Linux directory, domain, mail, gateway and infrastructure server for SMBs.

Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more.

A fully fledged, free email server solution, an open source project (GPL v2).