In a mixed Microsoft Exchange Server 2007 and Microsoft Exchange Server 2003 environment, an Exchange Server 2007 Client Access Server (CAS) server handles the proxy requests for Exchange Server 2003 servers. A user access a mailbox which is hosted on an Exchange Server 2003 server by using a third-party POP3 client. In this situation, after you apply Update Rollup 2 for Microsoft Exchange Server 2007 Service Pack 2 (SP2) on the Exchange Server 2007 CAS server, the user does not receive any new email messages by using the third-party POP3 client.
This issue occurs because authentication does not work well in the scenario that is described in the “Symptoms” section.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.