Ask anyone about what's new in SP2. You quickly get back the increased DB size limit, the mobility features, Sender ID, or the new Intelligent Message Filter version (IMF). Indeed these features are all important and will certainly bring about key improvements.
All those relying on IMF anti-spam protection have yet another reason for rejoicing. The improved filtering effectiveness will go beyond the new capabilities offered by SP2. It is not just because of Sender ID, or the built-in anti-phishing, or the brand new IMF version. Indeed you won't find this change on the SP2 feature list at all!
The BIG news is that IMF will be getting regular updates!! Twice a month, the signature databases for IMF anti-spam and anti-phishing will be updated. I came across this great news whilst watching a recent webcast from Alexander Nikolayev an Exchange Server Program Manager (see references).
As Alexander admits, the updates for the first IMF release were too few. The last update dates back to February 2005. This obviously was causing the signature database to get outdated as the spamming trends change continuously.
To be honest in my opinion this non-feature will have greater impact on IMF anti-spam protection than Sender ID, anti-phishing et al. Other anti-spam solutions always had the big advantage of being more dynamic than IMF. But now with signature updates every two weeks this is about to change.
This is not all, another very interesting point that comes out from the webcast regards false positives. Alexander states that the false positives rate with Exchange SP2 is in line with the industry average at approximately 10 to 15 out of every 100k messages. Alexander will post more details on this in a follow up blog.
I think most IMF users are aware of the anti-IMF campaigns being run by some third party anti-spam vendors. This campaign started off immediately following the first release of IMF. Many phantom highly exaggerated false positives rates are often quoted. So I am now really looking forward to Alexander's blog.
So if you missed it I highly recommend you to watch the webcast recording by following the link at the references. The webcast is a good tour of all new SP2 features including Sender ID, IMF improvements, troubleshooting event log errors, performance counters etc.
References
TechNet Webcast: A Comprehensive Look at Anti-Spam Technologies in Exchange 2003 SP2 (Level 300)