I don't live in the US, but if I receive an emvelope with my name and address on it I am allowed to read it.

And if in that envelope it says I opened a bank account, I am allowed to close it or at least ask the bank wtf they are doing.

Now I'd always argue for not acting destructively and be nice to people, e.g. assume they made a mistake and help them fix it. But if you are confusing my inbox with your own, you shouldn't be surprised if I read your mail. Mail that might expose other people's secrets.