Count # IDP addresses in 'to' line?

I am getting a fair amount of spam directly addressed to me. It often
comes as a single message addressed to several (~10) users at my ISP. The
ISP part is usually the same in each address(in this case, @netcom.com). 
Is there a way to add an 'ISP counter' for the To: line to my spam
routine? If this has already been done (I'll bet I'm not the first to want
one of these:-), please just give me a pointer. 

Thanks,

Larry Wright