
The reason I ask, is a guy got charged with "hacking" into his wife's e-mail account, even though he actually got in with a legit password, on his own computer no less. The password was found in the house somewhere. He suspected his wife was having an affair, it turns out he was right, and he's still being charged. I don't get it, he isn't wrong.

Morally he was wrong, the legality is pretty gray. Of course morally she was wrong too.

If the man and wife had been communicating via snail mail, and the wife had hidden the letters in a little box with a key, but left the box and key lying around, would the husband have been trespassing if he used the key to access the letters? Doesn't the law consider all matrimonial property to be joint property? If it is acquired during the marriage, it is jointly owned.
I think it's crappy that the wife had an affair, crappy that she hid it, crappy that the husband had to do whatever was in his means to figure out why his life was feeling screwed up, and in a way, it's really good that the husband finally had the facts and was able to make good decisions for himself without being lied to and manipulated by his wife. Bottom line, it's all crappy, but I don't think the husband should have legal problems because he accessed joint matrimonial property to protect himself from being screwed over by his wife.

I'm not sure if that's quite right, each case has to be judged on its own merits, maybe some people we don't like would agree to "play fair". I kind of like some of the laws of the Old West.

It is against the law for my husband to open my mail, why would my e-mail be any different?

Both my wife and I happen to have the same password for both our main email account. We are OK with opening each other's mail and have on occasion had her read a letter over the phone when I was out of town.
But normaly day to day we never do, although I wouldn't mind if she did as long as she used a knife or a letter opener to open it properly and left it in the appropriate place for me to read later.
She has a bad habit of tearing a letter open
If there is no trust in a marriage .....what is left...

If there is trust...there should be no need to open each other's mail, unless asked to do so...
Without trust, there just isn't much of a relationship.

You'd have to post a lot more details than that to expect any sort of coherent answer.
But let me ask you this: if you and your wife have separate bank accounts, is it fair to take her debit card and withdraw money, if you find the PIN number somewhere?

If there is trust...there should be no need to open each other's mail, unless asked to do so...
Without trust, there just isn't much of a relationship.