http://www.rfc-editor.org/rfc/rfc3656.txt
RFC 3656: The Mailbox Update (MUPDATE) Distributed Mailbox Database Protocol, R. Siemborski2003-12-01T23:00:00-00:00As the demand for high-performance mail delivery agents increases, it becomes apparent that single-machine solutions are inadequate to the task, both because of capacity limits and that the failure of the single machine means a loss of mail delivery for all users. It is preferable to allow many machines to share the responsibility of mail delivery. The Mailbox Update (MUPDATE) protocol allows a group of Internet Message Access Protocol (IMAP) or Post Office Protocol - Version 3 (POP3) servers to function with a unified mailbox namespace. This document is intended to serve as a reference guide to that protocol.