That may be how you feel about it, but the court may see it differently. If someone's paycheck is mistakenly mailed to you, even in cash, you can't deposit it.
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.
I agree that we should be mindful that there are many global legal traditions, but "unjust enrichment" is an English Common Law concept and much broader than American laws.
