IETF RFC 2221 PDF

When dealing with large amounts of users and many IMAP4 [RFC-2060] servers, it is often necessary to move users from one IMAP4 server to another. For example, hardware failures or organizational changes may dictate such a move.

Login referrals allow clients to transparently connect to an alternate IMAP4 server, if their home IMAP4 server has changed.

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.