In simple words, you need to create an XML file named MSExchange.UceContentFilter.xml and put it to the folder where MSExchange.UceContentFilter.dll is located (there's more then one, I'm not sure which is active just placed to all of them)
A sample of such file:
<?xml version="1.0" encoding="UTF-16"?&rt; <CustomWeightEntries xmlns="http://schemas.microsoft.com/2005/CustomWeight"&rt; <CustomWeightEntry Type="BODY" Change="-9" Text="important client's name"/&rt; <CustomWeightEntry Type="BOTH" Change="MIN" Text="important subject or name"/&rt; <CustomWeightEntry Type="SUBJECT" Change="MAX" Text="spam offer"/&rt; </CustomWeightEntries&rt;
To see, what actually SCL is assigned, add a custom form-field to MS Outlook, as described in this article.
Also, another good site about this.
It's probably possible to make Exchange IMF to add that SCL field to the message headers, so it could be viewed by a third party mail client.