Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Keycloak - Open Source Identity and Access Management for modern Applications and Services.
Roundcube - Web-based IMAP email client
Zentyal - Linux directory, domain, mail, gateway and infrastructure server for SMBs.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
OpenLDAP - OpenLDAP Software is an open source implementation of the Lightweight Directory Access Protocol.