The Stasi was able to read all mail of targets (and they had many) fifty years ago. Practically all mail crossing the border was read, which again was a lot. I'd assume addresses are scraped anyway, since they are digitized for routing already, and targeted mail being opened without leaving marks (an art existing for hundreds of years).
I wouldn't be surprised if it's possible to read mail while still sealed in an envelope. Wouldn't a sufficiently bright light and OCR be sufficient to read the average single page folded in thirds?
In 'Spycatcher' IIRC MI5 actually steamed letters open with a kettle, or rolled them out of the gap at the top of the flap with a split cane! They had no way (at that time) to deal with cellotape, however. I suspect the metadata is a pretty valuable resource in itself. Suggests that people wishing to protect their privacy would do well to shield the inside of their envelopes with junk text, tape them shut, post them from random locations and use mail forwarding services and a mailbox service
I imagine a terahertz imager would make short work of text on paper in a sealed envelope. Or a high-resolution IR camera, taking advantage of the emissivity difference between the ink and paper.
You could probably detect the message being opened if you wanted to. I think I have read somewhere about RFID to see if someone has opened a marketing mail, and I have also read about using eggs to seal the message to make it harder to open without destroying it entirely, as well as reusing the envelopes from junk mail to send your own messages. There is other stuff too possible such as trap messages I suppose. Also, the message may be hand written and is not necessarily typed, so hand written message would probably make OCR more difficult I should think.
