Cyrus IMAP is a free, open-source, self-hosted IMAP/JMAP mail server built at Carnegie Mellon and maintained with Fastmail. See features and alternatives.
Category
Communication
Pricing
Free / Open Source, from Free
Verified
Not yet
Last updated
July 18, 2026
Founded
1994
Headquarters
Pittsburgh, Pennsylvania, USA (Carnegie Mellon University)
Free PlanOpen SourceSelf-Hosted
Overview
Cyrus IMAP is a free, open-source mail server originating at Carnegie Mellon University in the early-to-mid 1990s, still hosted and governed there today, with the bulk of modern development driven by engineers at Fastmail, which uses Cyrus as its own commercial email backend.
It provides email, calendar (CalDAV) and contacts (CardDAV) service through a sealed-server security architecture, functioning as a self-hosted groupware backend comparable in scope to Microsoft Exchange.
Key Features
Cyrus supports IMAP, IMAPS, POP3, POP3S, KPOP and the modern JMAP protocol, along with native Sieve server-side filtering, mailbox access control lists, and per-user storage quotas.
Built-in CalDAV and CardDAV support lets Cyrus serve as a full calendar and contacts backend alongside email, distinguishing it from pure IMAP servers.
Pricing
Cyrus IMAP is free and open source under a BSD-style license, with no commercial pricing plans. Organizations bear only their own infrastructure and systems-administration costs.
There is no vendor selling Cyrus licenses; Fastmail contributes engineering time as a user of the software for its own hosted email service.
Key Features
IMAP/IMAPS, POP3/POP3S & JMAP Support — Supports legacy and modern mail access protocols, including Fastmail-driven JMAP development.
CalDAV & CardDAV — Native calendar and contacts support alongside email.
Sieve Server-Side Filtering — Standardized mail filtering that applies even when the client is offline.
Mailbox Access Control Lists — Fine-grained sharing and delegation permissions on individual mailboxes.
Storage Quotas — Per-user and per-hierarchy quota enforcement.
Sealed-Server Security Architecture — End users never get shell access to the mail server, improving security.
Large-Scale Mail Store — Optimized internal storage format built for very large institutional deployments.
Active JMAP Development — Ongoing investment in the modern JMAP protocol in partnership with Fastmail.
Pros & Cons
Pros
Free, open source, and proven at large institutional scale for decades
Native calendar and contacts (CalDAV/CardDAV) support beyond just email
Strong security model via sealed-server architecture
Actively maintained with modern JMAP protocol investment via Fastmail
Cons
Steep setup and administration learning curve; not beginner friendly
No built-in web interface; requires a separate webmail client
Smaller self-hosting community than Dovecot for small deployments
Internal mail-store format is less portable than standard Maildir/mbox
Pricing
Community Free N/A
Frequently Asked Questions
Is Cyrus IMAP free?
Yes, Cyrus IMAP is free and open source under a BSD-style license.
Who maintains Cyrus IMAP?
Carnegie Mellon University originated and continues to host the project, while Fastmail engineers drive much of the modern development.
Does Cyrus IMAP support calendars and contacts?
Yes, via native CalDAV and CardDAV support alongside email.
What's the difference between Cyrus IMAP and Dovecot?
Cyrus uses a sealed-server model geared toward large institutional deployments, while Dovecot is generally easier to configure and more common in small-to-mid-size self-hosted setups.
Does Cyrus support JMAP?
Yes, Cyrus has ongoing active development of the JMAP protocol, co-driven by Fastmail.
Who uses Cyrus IMAP?
Universities, ISPs and enterprises that need to self-host mail infrastructure at scale, including Carnegie Mellon University and Fastmail.