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Microsoft Exchange Server 2007 Service Pack 1 (SP1) Rollup 2 (RU2) provides support for PLAIN authentication in both POP3 and IMAP4 services. However, PLAIN authentication cannot be disabled. Therefore, POP or IMAP users who use PLAIN authentication will be unable to connect to back-end Microsoft Exchange Server 2003 mailbox servers through Exchange 2007 CAS.
For more information about Update Rollup 2 for Exchange Server 2007 Service Pack 1, see the following Exchange Help topic:
After you apply this rollup, when a POP or IMAP user sends PLAIN authentication requests, Exchange 2007 servers will use a proxy server and LOGIN authentication to connect to Exchange Server 2003 back-end servers.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about PLAIN authentication, visit the following RFC Web sites: