Opening Mail With Someone Else's Name

[celais]

I'm curious if someone can really get in trouble if they open mail that doesn't have there name on it?

Are you supposed to send it back to the sender and can you get in trouble for throwing it away {if that's what the person claims they did}
I have proof that mail with my husbands name from our insurance company has been going to his ex-wifes address. I found out from the "online version" of the bill...only thing is they are not getting forwarded back or forwarded to us. Can she get in trouble?

ETA - They are using her address because thats what she is using as billing claim address when she takes child to the doctor ....so then the claim is getting sent to her address with my husbands name clearly spelled out