It’s called “Informed Delivery”-- when you sign up, the US Postal Service will email you images of your letter-sized mail each morning.
Did you know about this? It could be convenient, especially when you're spending time away from home. And no, USPS doesn't open the mail; they take photos of the outside of envelopes. The service is currently available in just a few parts of the US, and the DC area is included.