The Cyrus IMAP Server is a scaleable enterprise mail system
designed for use from small to large enterprise environments using
standards-based technologies.

A full Cyrus IMAP implementation allows a seamless mail and bulletin
board environment to be set up across multiple servers. It differs from
other IMAP server implementations in that it is run on "sealed"
servers, where users are not normally permitted to log in. The mailbox
database is stored in parts of the filesystem that are private to the
Cyrus IMAP system. All user access to mail is through software using
the IMAP, POP3, or KPOP protocols. TLSv1 and SSL are supported for
security.

This is the main package, install also the cyrus-imapd-utils package (it
contains server administration tools and depends on the perl-Cyrus
package).