Sorry, Navy, I misread your post. You didn't claim it was mail fraud.
I googled around a bit and the best information I found so far was here:
What is the punishment for going into someone else's email?
It's not exactly clear, but I don't see how he'd fit under the first definition, as he didn't hack into someone's computer.
The third sounds the closest to what he did, though technically he didn't happen upon Palin's computer but she stupidly made it easy for anyone to figure out her password.
It'll be interesting to see what they do with this.