Top 20 products relevant to Email, Self-hosted Email, and Email Clients

Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.

Open Source

Web-based IMAP email client.

Open Source

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

Open Source

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

Open Source

Full featured email server built in docker with free plan.

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

Mailu is a simple yet full-featured mail server as a set of Docker images.

SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.

Cal and CardDAV server.

Open Source

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

Open Source

A personal server for owning your email and more.

Sharing & communication software for business.

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

DAViCal is a server for calendar sharing.

Lightweight CalDAV/CardDAV server on top of Git.

Open Source

OBM is a free messaging and collaboration platform useful for just a couple to many thousands of users with strong support for mobile devices.

Simple and easy-to use PHP e-mail client.

CalendarServer is a standards-compliant server implementation of the CalDAV and CardDAV protocols, including iMIP and APNS.

AgenDAV is an Open Source CalDAV Web application.

grommunio (formerly known as grammm) efficiently combines all requirements of modern, digital communication and collaboration.