The Cyrus IMAP (Internet Message Access Protocol) server provides access to
personal mail and system-wide bulletin boards through the IMAP protocol.
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.

The private mailbox database design gives the server large 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.

WWW: https://www.cyrusimap.org/