What are some alternatives?
G Suite - Google's encompassing suite of cloud-based business apps.
Zimbra Collaboration Suite - Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more.
Zoho - Zoho is an all-in-one online office suite.
Kerio Connect - KerioConnect is a commercial mail and groupware server.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Slack - A messaging app for teams who see through the Earth!