Now, did you want that in terms of "number of copies installed" or "amount of mail handled"? There's probably zillions of little Fedora and Ubuntu boxes running whatever MTA came off the disk that are handling 1 or 2 pieces of mail a day, and then there's whatever backends are used by MSN/Hotmail, Yahoo, AOL, etc. "This MTA packed by weight, not by volume. Some settling of contents may have occurred during shipping and spamming."
(Seriously - if 95% of the mail out there is spam, then the top 4-5 MTAs are probably the ratware that's sending out the spam. Something to consider...)
In keeping with this concept, and turning it around. What MTA is exposed to the most spam? (1-x) That should tell you what MTA handles the most "good" mail by also being the destination for the most spam (good, live recipients). Or I could be missing something well known about mail flows. Deepak