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.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Google Workspace - Google's encompassing suite of cloud-based business apps.
Roundcube - Web-based IMAP email client
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Exchange - On-demand technical interview coaching by engineers