Re: Capital One
If the post is addressed to your address then I am sure that under the Postal Sevices Act you would not be liable if you opened it in order not to detriment the intended recipient.
So if you opened a handwritten addressed envelope with no return address to contact the company on the letterhead you are not acting in detriment of the two parties.
Saying that, when Barclays started sending demands for cash from the person who sold us our house, when we read them the account number over the phone we were accused of illegal tampering of mail.
Except I was reading it through the Window of the envelope in clear view for all to see.