Microsoft's Intelligent Message Filter (IMF) add-on for Exchange Server 2003 is one of the niftiest free downloads you can put in your Exchange organization that will have direct effects your users can see. However, I've seen a few questions and issues popping up regarding IMF that tell me there's a mild bit of confusion out there. Let's straighten it out.
There are two versions of IMF. IMF version 1 is the version you can download separately. It's got the basic message filtering goodness built in, but lacks a lot of other nice features. IMF version 2, on the other hand, comes with Exchange Server 2003 SP2; you can't download it separately and you can't choose it as an optional component. It will automatically get installed when you apply SP2. Please note that you cannot downgrade to IMFv1 after installing SP2 and have things keep working. If you do install IMFv1 on a server that already has SP2 applied, you'll need to reinstall SP2.
IMF doesn't automatically upgrade from v1 to v2. It would have been nice if the SP2 installer did this for you, but you must manually uninstall IMFv1 from your Exchange server before you upgrade it to SP2.
IMFv2 is not automatically enabled. No matter your config before SP2, IMF will not be enabled by default -- which makes sense, since you don't need it running on every server in your organization. Happily, it's easy to enable it on a per-virtual server basis. You do it from the same property page that you enable connection filters, recipient filters, and sender filters from on the SMTP virtual server. Heck, you also enable Sender ID here.
IMFv2 is worth upgrading to. If you haven't upgraded to SP2 yet, why not? Beside all the other great features, IMFv2 includes built-in Sender ID integration, the ability to modify the word list so you can customize the filter, and auto-update capabilities.
Update: I talked a bit about some other IMFv1 to IMFv2 upgrade issues in my earlier post about SP2, so check it out too.