Cyrus IMAP

Cyrus IMAP provides a self‑hosted server for email, contacts, and calendar data, exposing IMAP, POP3, JMAP, CalDAV, CardDAV, NNTP and WebDAV interfaces. Mailboxes are stored in a private filesystem area, allowing multiple concurrent read/write connections, access‑control lists and hierarchical quotas. The system includes built‑in support for spam and virus scanning, SPF/DKIM/DMARC validation, and a flexible Sieve filtering language.

The server targets organizations that need a secure, scalable solution without user‑level system logins, such as universities and enterprises. It runs on Linux and is packaged by most major distributions, while also offering source tarballs and a Git repository for custom builds. Documentation covers installation, configuration, replication and advanced features.

Cyrus is released under a BSD‑3‑Clause license, is marked as stable for the 3.12 series, and continues active development since 1993, with a focus on efficiency, extensibility and adherence to open standards.