Cyrus IMAP

Cyrus IMAP is a long-running, production-grade server for hosting email along with contacts and calendars. It focuses on security and scalability by keeping mailboxes in server-managed storage and exposing access via standard protocols.

Key Features

• IMAP and JMAP support for mailbox access
• CalDAV and CardDAV support for calendars and contacts
• POP3, NNTP, and WebDAV protocol support (deployment-dependent)
• Mailbox ACLs and quota management for multi-user environments
• Replication and “Murder” aggregation for large and distributed setups
• Flexible server-side mail filtering via Sieve
• Search capabilities (with optional components depending on build)
• Extensive authentication options via Cyrus SASL
• Integrations for antivirus and antispam toolkits
• Deliverability-related support for SPF, DKIM, and DMARC (typically via integration with surrounding mail stack)

Use Cases

• Hosting IMAP/JMAP mailboxes for an organization where users do not need shell access
• Providing CalDAV/CardDAV services for groupware-style deployments
• Building a scalable mail backend for service providers or large enterprises

Limitations and Considerations

• A full email system typically requires integration with an MTA and additional components for webmail, spam filtering, and DKIM signing

Cyrus IMAP is well-suited to administrators who need a robust standards-based messaging platform with strong performance characteristics and operational controls. It is commonly used in demanding environments where scalability, manageability, and protocol support are priorities.