CDRLabs.com

Bill Gates, not his company, receives about 4 million pieces of junk e-mail each day!
However, special anti-spam technology limits what actually makes it to his inbox...

http://www.cnn.com/2004/TECH/internet/11/18/gates.spam.ap/index.html

Yeah, "special anti-spam" technology sounds like 500 people reading stuff and clearing out the crud. I wouldn't get much spam either with a team of people skimming through my emails.

another explanation (more likely):
yeah, M$ does make good software, spam filtering in this instance. it's just that they keep it for in-house use only.
for outside (=clients who buy their software) they produce the crap we all know and love.