Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Keycloak - Open Source Identity and Access Management for modern Applications and Services.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
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.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.