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In Exchange 2007 Service Pack 1 (SP1) Rollup 4 (RU4), you cannot log on to a hidden mailbox by using Base64 encoded PLAIN authentication through IMAP4 or POP3 protocols. The same operation has no problem in Exchange 2007 SP1 RU3 or in earlier versions.
Note You can identify this problem if you telnet to the Exchange Server 2007 SP1 server and issue the following commands:
a2 Authenticate PLAIN
<base64 encoded authentication string>
When this problem occurs, you receive a "NO AUTHENTICATE failed" error message. The usual returned message should be "OK AUTHENTICATE completed".
There is a Lightweight Directory Access Protocol (LDAP) query during the authentication process in which hidden mailboxes are missed.
To resolve this problem, install Update Rollup 9 for Exchange 2007 Service Pack 1. For more information about Update Rollup 9 for Exchange Server 2007 Service Pack 1, see the following Exchange Help topic:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
You can generate the Base64 encoded authentication string in Exchange Management Shell by running the following script. However,you must replace the domain, the username, and the real_password fields with the appropriate domain, username, and password: