Opening someone else's mail is actually illegal, so if you believe that laws are in place to protect moral guidelines then it's the same thing.
It doesn't matter whether they find out or not - chances are, you'll start treating the person differently once you've read their emails. The damage is already done.
Whatever you tell yourself to justify it, it is an invasion of privacy and you wouldn't want someone else to do it to you - it's not as bad as breaking into their house to snoop through all their stuff, but morally it's not far off.
Why do you want to do this anyway? Can't you just ask them whatever it is you need to know?