Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.