Cyrus IMAP is an open-source email, contacts and calendar server. Users reach their data through JMAP, IMAP, POP3, CalDAV, CardDAV and WebDAV. The software also provides Sieve filtering, replication, SPF, DKIM and DMARC handling, and authentication through Cyrus SASL. It is designed for large installations and for sealed servers on which ordinary users cannot log in. Cyrus IMAP is installed and operated by the organisation itself.
The project began at Carnegie Mellon University in the United States. There is no controlling company: according to the bylaws, a Governance Board and a group of core developers set the direction. The source code is on GitHub. Because you run the server yourself, your organisation decides where the data is stored, for example in the EU. Support comes from the community and mailing lists; there is no commercial vendor as a contracting party. This means full control over the data, together with responsibility for operating the system.