IETF RFC 2193 PDF

When dealing with large amounts of users, messages and geographically dispersed IMAP4 [RFC-2060] servers, it is often desirable to distribute messages amongst different servers within an organization. For example an administrator may choose to store user's personal mailboxes on a local IMAP4 server, while storing shared mailboxes remotely on another server. This type of configuration is common when it is uneconomical to store all data centrally due to limited bandwidth or disk resources.

Mailbox referrals allow clients to seamlessly access mailboxes that are distributed across several IMAP4 servers.

A referral mechanism can provide efficiencies over the alternative "proxy method", in which the local IMAP4 server contacts the remote server on behalf of the client, and then transfers the data from the remote server to itself, and then on to the client. The referral mechanism's direct client connection to the remote server is often a more efficient use of bandwidth, and does not require the local server to impersonate the client when authenticating to the remote server.