Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.

Roundcube - Web-based IMAP email client

Microsoft 365 - Boost your productivity with reliable access anywhere with services like email, calendar, file sharing, meetings, instant messaging, and Office Online

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

Envigeek Digital Workspace - Business email can be more than just a mailbox. It can be a virtual office where all the team work and online meetings happen. You can create and edit documents there – alone, or in collaboration with your team.

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