The Cyrus IMAP server differs from other IMAP server implementations in that it is generally intended to be run on sealed servers, where normal users cannot log in. The mail spool uses a filesystem layout and format similar to the Maildir format used by other popular email servers such as qmail, Courier, Dovecot, etc. Cyrus IMAP's spool format, said to have improved performance and scalability to Maildir, resides in parts of the filesystem that are private to the Cyrus IMAP system. Users can access mail through the IMAP/IMAP-S, POP3/POP3-S or KPOP protocols.
The Cyrus IMAP server supports server-side mail filtering through the implementation of a mail filtering language called Sieve.
The private mailbox database design gives the server considerable advantages in efficiency, scalability, and administratability. Multiple concurrent read/write connections to the same mailbox are permitted. The server supports access control lists on mailboxes and storage quotas on mailbox hierarchies.