Actually, I've read a number of things by Microsoft employees and that email is exactly like something Gates would say. Here's an article where the writer says Gates said this:

As for the message, Gates smiled and said, "There's not a day that I don't send a piece of e-mail ... like that piece of e-mail. That's my job."

Gates is apparently very direct in his criticism of employees. The employees who have meetings with Gates and don't get berated take that as a badge of honour. Here's a comment by Joel Spolsky:

Later I had it explained to me. "Bill doesn't really want to review your spec, he just wants to make sure you've got it under control. His standard M.O. is to ask harder and harder questions until you admit that you don't know, and then he can yell at you for being unprepared. Nobody was really sure what happens if you answer the hardest question he can come up with because it's never happened before."

So I don't know for sure if Gates wrote the email in question, but he could have.