I received this nice e-mail asking about creating rules on more message properties than we support today in OWA 2003.  Since Exchange has been around a long time and I've only been part of the team for one release, I thought I'd share when I learn something new as well.

Hello,
Thanks for publishing this blog. I found answers to some of the questions I had about OWA and exchange 2003.
I wonder if you can give some help about one specific issue I am dealing with OWA 2003. We are about to launch hosted Exchange services and some of our clients will have only OWA to access their accounts. In OWA, there is no option to create a rule using a keyword in the message header. Is there a way to do that or is there a modified control somewhere with more ways to create rules ?
Thank you
Turker Sokullu
Canada

There's a Rules COM component you might find useful for creating rules to act on additional message properties that is part of the Exchange 5.5 platform SDK. 
 
Here is a KB article describing how to create a rule with the component in VBA:
 
Here is more information about the component in MSDN:
 
Hope that helps.