Couple of questions:
Do you know he may find out about it? For example, if the sender sent it return receipt and you pick it up at Noon and she gets the receipt saying that he got it yet when she tells him he says I didn't read my mail.
Why do you want to read it? Nosy, afraid he's cheating, etc.
Is there anything else going on, like a divorce? How might it look to a judge or other party if it comes out.
The odds of you being criminally prosecuted are rather low, particularly if you claim he asked that you check his email, but the benefits may be lower. And it makes a sad statement about your marriage.