Interestingly, the USPS website used to say that it was, but ultimately, it's a judge who determines if something is legally binding. When the ERR first came out, some judges didn't accept it as carrying the same factual weight as the green card, but that was probably 20 years ago now. These days, most courts recognize ERR as a valid proof of delivery, you should always go with the way that you believe will protect you the most if you believe you'll be in front of a court discussing Certified Mail or proof of delivery.