MUPDATE — the distributed mailbox-database protocol that keeps a Cyrus Murder mail cluster's namespace consistent.
RFC 3656 defines the Mailbox Update protocol and IANA registered 3905 for it. In a Cyrus Murder, several IMAP/POP backends share one mailbox namespace: an MUPDATE master holds the authoritative mailbox database and makes operations on it atomic, while frontends and slaves connect to replicate it. The protocol is line-based and text-like, and a client authenticates over SASL after the server advertises its mechanisms. A listener on 3905 means a multi-server Cyrus mail cluster, not a single mail host.
Do not expose
The MUPDATE master is the mail cluster's source of truth for who owns which mailbox — a strictly internal, backend-to-backend service that should be confined to the cluster's own network.
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