Unsolicited e-mail is being delivered to Exchange server mailboxes.
Cause: Connection filtering is not configured on your Exchange server.
Solution: Exchange 2003 supports connection filtering based on block lists, which are lists that can be queried by your Exchange server to identify verified spam sources. Connection filtering uses external services that list known sources of unsolicited e-mail, dial-up user account lists, and servers open for relay based on IP addresses on block lists that they maintain. Connection filtering complements third-party content filter products. You can also configure connection filtering without using a block list provider by creating global accept and deny lists of SMTP addresses from which you want to globally accept or deny all e-mail.
To configure connection filtering, you must first create and configure a connection filtering rule, and then apply it your SMTP virtual server. For more information, search for "Configure Connection Filtering" in Exchange server Help. To access Exchange server Help, click Start, click Server Management, and then press F1.