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.
Google Workspace - Google's encompassing suite of cloud-based business apps.
mailcow - An open source mailserver suite.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Roundcube - Web-based IMAP email client
Kerio Connect - KerioConnect is a commercial mail and groupware server.