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.
To resolve this problem, install the following update rollup:
2279665 Description of Update Rollup 1 for Exchange Server 2007 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.