He said he used to get a lot, but now it's down to 60 to 70 pieces a day. And he said he has no personal assistants filtering his email.

"Why do people get a lot of mail? They get a lot of mail because they sign up to get a lot of mail and they get spammed by a lot of mail, but people don't waste other people's time generally," he said.

He regularly shares his e-mail address publicly -- "I'm steveb@microsoft.com," Ballmer said -- and then acknowledged that tomorrow there might be more than 60.

Ballmer said the e-mails he does get are typically "thoughtful suggestions, improvements, things we should be doing differently." And usually he responds himself or forwards them to someone at Microsoft to respond.

What keeps him going now after all these years and billions in the bank?

Ballmer said he enjoys the chance "to really be at the forefront of changing the world" and working with the smartest people in the world. He also loves the challenge.

"With the scale and magnitude of what's going on in our industry, and what we're trying to do and the guys we have to compete with, I couldn't just sit at home and work on my golf game or something. I like big challenges and we've got 'em."

Check out our complete coverage of Ballmer's Q&A with Guy Kawasaki.