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.
mailcow - An open source mailserver suite.
Google Workspace - Google's encompassing suite of cloud-based business apps.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.