I'm reading up more on Exchange and becoming familiar with some of the options the product offers.  The article below discusses the Sender ID filtering option, it also shows how to apply various policies on incoming SMTP messages. Here are some good reference links besides http://Microsoft.com/exchange/

Various Exchange tools (misc stuff these are free)
http://www.microsoft.com/technet/prodtechnol/exchange/downloads/2003/tools.mspx

Configuring and enabling Sender ID filtering in Exchange 2003 SP2
http://www.msexchange.org/tutorials/Configuring-enabling-Sender-ID-filtering-Exchange-2003-SP2.html

Good Exchange website (tons of articles).
http://www.msexchange.org

ListServ (pretty busy but good info)
http://www.webelists.com/cgi/lyris.pl?enter=exchangelist

Various Exchange info
http://www.msexchange.org/pages/links.asp#Exchange

Steve