Meaning “yes, I received your envelope, but the letter is not what you say”? There’s nothing to prevent that. On the rare occasion when I worry about that possibility, I take pictures, at the post office, of the letter, the envelope, and the Certified Mail label, and then the letter being inserted in the envelope. I’ve never had to use the photos, but if it actually went to court, the argument is generally “Why would I NOT include the right letter? There’s no advantage in that. And let them show exactly what they received.” Judges have seen all the excuses, and I’m sure “I received a blank piece of paper!” is an old one that gets waved away dismissively.