Research question: How many legitimate mail sources are out there?

Would someone out there please take a stab at answering this question?  
Or has it been answered already?

What to I mean by a 'mail source'? Well, in technical terms I'm thinking 
someone could say something like:
"Here's info from a large ISP (handling mail for x million customers).
Over a period of ____ it accepted mail from x IP addresses that it 
thinks were sending it mostly ham"

I'd be even happier with something like this:
it accepted mail from x TLD Names in the HELO.

I could live with something like: my greylisting server whitelisted x IP 
addresses (I'm figure John Levine could give us this number quite 
quickly for his a
"several hundred individual mailboxes, a few dozen mailing lists, and 
the abuse.net message forwarding system.")

Of course, sometimes several servers send through one IP, or one server 
sends through several IPs...

I'm guessing that there are under a million such mail sources.  There 
are around .1 billion TLD Names*.

This is apropos this thread: SPF Loses Mindshare at
http://www.circleid.com/article.php?id=1157_0_1_0_C/

*http://www.registrarstats.com/zonefile_enlarged.asp (Could be a few 
times more more, as this doesn't list ccTLDs...)