IMAP4 Mailbox Referrals
RFC 2193

Document Type RFC - Proposed Standard (September 1997; No errata)
Updated by RFC 5258
Author Mike Gahrns 
Last updated 2013-03-02
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Network Working Group                                        M. Gahrns
Request for Comments: 2193                                   Microsoft
Category: Standards Track                               September 1997

                        IMAP4 Mailbox Referrals

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

1. Abstract

   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

2. Conventions used in this document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.

   A home server, is an IMAP4 server that contains the user's inbox.

   A remote mailbox is a mailbox that is not hosted on the user's home

Gahrns                      Standards Track                     [Page 1]
RFC 2193                IMAP4 Mailbox Referrals           September 1997

   A remote server is a server that contains remote mailboxes.

   A shared mailbox, is a mailbox that multiple users have access to.

   An IMAP mailbox referral is when the server directs the client to
   another IMAP mailbox.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC-2119].

3. Introduction and Overview

   IMAP4 servers that support this extension MUST list the keyword
   MAILBOX-REFERRALS in their CAPABILITY response.  No client action is
   needed to invoke the MAILBOX-REFERRALS capability in a server.

   A MAILBOX-REFERRALS capable IMAP4 server MUST NOT return referrals
   that result in a referrals loop.

   A referral response consists of a tagged NO response and a REFERRAL
   response code.  The REFERRAL response code MUST contain as an
   argument a one or more valid URLs separated by a space as defined in
   [RFC-1738]. If a server replies with multiple URLs for a particular
   object, they MUST all be of the same type. In this case, the URL MUST
   be an IMAP URL as defined in [RFC-2192].  A client that supports the
   REFERRALS extension MUST be prepared for a URL of any type, but it
   need only be able to process IMAP URLs.

   A server MAY respond with multiple IMAP mailbox referrals if there is
   more than one replica of the mailbox.  This allows the implementation
   of a load balancing or failover scheme. How a server keeps multiple
   replicas of a mailbox in sync is not addressed by this document.

   If the server has a preferred order in which the client should
   attempt to access the URLs, the preferred URL SHOULD be listed in the
   first, with the remaining URLs presented in descending order of
   preference.  If multiple referrals are given for a mailbox, a server
   should be aware that there are synchronization issues for a client if
   the UIDVALIDITY of the referred mailboxes are different.

   An IMAP mailbox referral may be given in response to an IMAP command
   that specifies a mailbox as an argument.

Gahrns                      Standards Track                     [Page 2]
RFC 2193                IMAP4 Mailbox Referrals           September 1997


      A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/REMOTE]Remote Mailbox

   NOTE: user;AUTH=* is specified as required by [RFC-2192] to avoid a
   client falling back to anonymous login.

   Remote mailboxes and their inferiors, that are accessible only via
   referrals SHOULD NOT appear in LIST and LSUB responses issued against
   the user's home server.  They MUST appear in RLIST and RLSUB
   responses issued against the user's home server. Hierarchy referrals,
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