I never built mail sorting equipment, but I did work with that camera quite a bit. It has very good sensitivity. For a lark one day, I decided to see if I could read mail through the envelope. So, I fed some junk mail in front of it and found that for non-privacy type envelopes, it was almost as easy to read what's inside (top page) as what was on the outside of the envelope. Multiple pages stacked and folded made it impossible to read everything, but you could have read the outer page portion of a tri-folded letter on each side. Keep in mind that I implemented this for fun on a slow afternoon (and it required no special wizardry). Given time and resources, maybe a second camera or different lighting, a lot more might be achieved. Even a naive approach of scanning a letter multiple times under different lighting conditions might be enough to read a large fraction of letter mail. It would take less than a minute. It would not require opening or touching the letter more than an ordinary mail sorter.

Now I am not alleging that I think that all mail is stored/read, but I am saying that the tech needed to do that to some degree exists and is at least partially in place.