I'll post the answer in 48 hours, but for now, take a stab without googling. The total number of worldwide email accounts is approx...

Appx 300+ million US valid addys. Appx 300+ million Non-US valid addys. Just off the top of my head. Am I close?

The answer... is 3.4 billion. I had documented where I found this in an article but dang it, I can't find it. You'll have to trust me.

The right answer is infinity because of catch-all accounts, not to mention "+" extensions and other tricks to make one valid account addressed via multiple ways...