XCON: IMAP4 Referral Support

This article explains IMAP referral as it relates to Microsoft's current IMAP clients and Exchange Server, and future plans for these products.

IMAP referral is the process where, initially, an IMAP client issues a logon command to an IMAP server that is not the IMAP client's home server. The IMAP server passes the client's logon command to another IMAP server that can authenticate the client. IMAP referrals happen behind the scenes and are transparent to the user.

Exchange Server version 5.5 supports referrals, but not completely. You will be referred to the correct server, but you will not be able to retrieve mail. For automatic referrals to work properly, Exchange Server computers that will participate in this referral "scheme" must be configured to do directory replication amongst themselves. After the Exchange Server computers have replicated successfully, all IMAP logon requests will be "referred" to the appropriate IMAP server by means of a directory lookup.

The long-term plan is to have Exchange Server be a "front-end" for the referral for non-supporting clients, meaning that a single properly configured Exchange Server computer would transparently handle the referral for the non-referral IMAP clients. At this time, front-end referral functionality is not available in Exchange Server 5.5 and earlier versions, but will be included in the next release of Exchange Server.

Unfortunately, Post Office Protocol 3 (POP3) does not have the ability to do referral request and responses. Also, Outlook Express and other Microsoft IMAP clients do not support referrals and are not expected to support referrals in the future release.

Below are some instructions on the protocol commands that you can use in a telnet session to see if the IMAP server supports referrals:

1. In Windows NT 4.0, click Start, and then click Run.
2. In the Open text box, type telnet <SECOND> 143, and press ENTER.
   
   NOTE: <SECOND> represents the name of the second server; do not include the angle brackets (<>).
   
   You should see a new window appear with a banner that looks similar to the following:
   * OK Microsoft Exchange IMAP4rev1 server version 5.5.1960.6 (reedlm4.com) ready
3. On the Terminal menu, click Preferences. Make sure the Local Echo check box is enabled, and then click OK.
4. At the prompt, type ? login <NTdomain>/<domain userID>/<Exchange Alias> <password>, and press ENTER. For example, if your Windows NT domain is DOMname, your domain user is UserN, your Exchange alias is AliasN, and your password is PASSword, then you would type:
   ? login DOMname/UserN/AliasN PASSword
   
   NOTE: The the direction of the slashes is very important; the question mark, followed by a space at the beginning of the line is also important. Backspacing is not allowed; if you make a mistake, you have to press ENTER, and start a new line.
   
   
5. The response you will get will be something like:
   ? OK [REFERRAL imap://redmond%2Freedl%2Fimapf;AUTH=*@reedlm1.reedlm1.com] LOGIN completed
6. At this point, you are logged onto the other IMAP server. When you issue any commands, such as ? Select Inbox, to enter into your Inbox, you may receive the following message:
   NO There is no replica for that mailbox on this server.
7. You can log out of the telnet connection by typing ? logout at the command prompt, and pressing ENTER.