IMAP4 Login Referrals
RFC 2221

Document Type RFC - Proposed Standard (October 1997; No errata)
Author Mike Gahrns 
Last updated 2013-03-02
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Network Working Group                                           M. Gahrns
Request for Comments: 2221                                      Microsoft
Category: Standards Track                                    October 1997

                         IMAP4 Login 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.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

1. Abstract

   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

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 server is a server that contains remote mailboxes.

Gahrns                      Standards Track                     [Page 1]
RFC 2221                 IMAP4 Login Referrals              October 1997

   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
   LOGIN-REFERRALS in their CAPABILITY response.  No client action is
   needed to invoke the LOGIN-REFERRALS capability in a server.

   A LOGIN-REFERRALS capable IMAP4 server SHOULD NOT return a referral
   to a server that will return a referral. A client MUST NOT follow
   more than 10 levels of referral without consulting the user.

   A LOGIN-REFERRALS response code MUST contain as an argument a valid
   IMAP server URL as defined in [IMAP-URL].

   A home server referral consists of either a tagged NO or OK, or an
   untagged BYE response that contains a LOGIN-REFERRALS response code.

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

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

4. Home Server Referrals

   A home server referral may be returned in response to an AUTHENTICATE
   or LOGIN command, or it may appear in the connection startup banner.
   If a server returns a home server referral in a tagged NO response,
   that server does not contain any mailboxes that are accessible to the
   user.  If a server returns a home server referral in a tagged OK
   response, it indicates that the user's personal mailboxes are
   elsewhere, but the server contains public mailboxes which are
   readable by the user.  After receiving a home server referral, the
   client can not make any assumptions as to whether this was a
   permanent or temporary move of the user.

4.1.  LOGIN and AUTHENTICATE Referrals

   An IMAP4 server MAY respond to a LOGIN or AUTHENTICATE command with a
   home server referral if it wishes to direct the user to another IMAP4

   Example:  C: A001 LOGIN MIKE PASSWORD
             S: A001 NO [REFERRAL IMAP://MIKE@SERVER2/] Specified user
                     is invalid on this server. Try SERVER2.

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RFC 2221                 IMAP4 Login Referrals              October 1997

             S: A001 OK [REFERRAL IMAP://MATTHEW@SERVER2/] Specified
                     user's personal mailboxes located on Server2, but
                     public mailboxes are available.

             <authentication exchange>
             S: A001 NO [REFERRAL IMAP://user;AUTH=GSSAPI@SERVER2/]
                     Specified user is invalid on this server. Try

4.2. BYE at connection startup referral

   An IMAP4 server MAY respond with an untagged BYE and a REFERRAL
   response code that contains an IMAP URL to a home server if it is not
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