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One potential issue to be aware of with Exchange 2003 SP2 is that you will have to reconfigure your IMF settings if you already have that component installed. If version 1 of the IMF is installed prior to running SP2 setup, you may receive an error message during the install that tells you that you have to uninstall IMF v1 first before completing the service pack install. IMF v2 is automatically included as part of the SP install.
Once SP2 is installed, you then have to reenable the IMF in both message delivery (Exchange System Manager -> Domain name (Exchange)-> Global Settings -> Message Delivery -> Right-click, properties -> Intelligent Message Filtering) and on the SMTP virtual Server (Exchange System Manager -> Domain Name (Exchange) -> Servers -> Server Name -> Protocols -> SMTP -> Default SMTP Virtual Server -> right-click, properties) -> General tab -> 'Advanced' button -> Edit -> Apply Intelligent Message Filter).
The SP2 release notes call this out in more detail (although it can be easy to miss all of the details):
Microsoft Exchange Intelligent Message Filter is integrated with the Sender ID feature in the Exchange Server 2003 Service Pack 2 (SP2). Note that, for Exchange Server 2003 SP2, Intelligent Message Filter is built into the product and is considered to be version 2. Version 1 of the Intelligent Message Filter was a stand-alone tool.
Intelligent Message Filter helps companies reduce the amount of unsolicited commercial e-mail (UCE), also know as spam, that can be received by users.
If Intelligent Message Filter version 1 is detected as being on the target server computer when you start the upgrade to SP2, a flag is raised and you must uninstall version 1. After the SP2 upgrade, Intelligent Message Filter version 2 is automatically installed on the server. However, the administrator still must enable version 2 on the Virtual Server Instance (VSI).
Note: Do not install Intelligent Message Filter version 1 after the upgrade to Exchange Server 2003 SP2. Version 2 has been installed automatically with SP2.
For information about Version 1 of Intelligent Message Filter, see the Microsoft Exchange Intelligent Message Filter Deployment Guide.
During the upgrade to Exchange Server 2003 SP2, the server will not be available. When the server becomes available again, the administrator must enable Intelligent Message Filter immediately. Or, the server must be taken offline and Intelligent Message Filter enabled, if more time is required. Otherwise the server will be subject to spam attacks.