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.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Kerio Connect - KerioConnect is a commercial mail and groupware server.
mailcow - An open source mailserver suite.
Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.