Certain third-party IMAP4 clients cannot connect to Exchange Server 2003 mailboxes through an Exchange Server 2010 CAS server

In a mixed Microsoft Exchange Server 2003 and Microsoft Exchange Server 2010 environment, the Exchange Server 2003 mailboxes are accessed through an Exchange Server 2010 Client Access Server (CAS) server. In this case, users cannot access their Exchange Server 2003 mailboxes by using certain third-party IMAP4 clients. Additionally, the users may receive the following error message:

This problem occurs because certain third-party IMAP4 clients use the PLAIN authentication mechanism. The Exchange Server 2010 CAS server performs an additional check on the namespace in the Exchange Server 2003 server. However, the responses from the Exchange Server 2003 server are mishandled by the Exchange Server 2010 CAS server. Therefore, users cannot access their Exchange Server 2003 mailboxes by using certain third-party IMAP4 clients.

To resolve this problem, install the following update rollup:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

For more information about the IMAP4 protocol and the PLAIN authentication mechanism, visit the following IETF Web sites: