Gmail vs Hotmail vs Yahoo Mail: Who's on first?

We know there is a hotly contested race for the most used browser, with each party claiming the most users, but now the email providers are getting their licks in also. I guess the final tally and the ultimate winner really depends on whose creative math you use for the web-based email service numbers total. Yahoo and Microsoft said they use ComScore to measure their numbers. Google said it does not comment on numbers generated by third parties.