[scrabble]

Could you not just refrain from writing a return address on your mail? Or instead, put down a false return address?

In that case there'd be no tracking of who is mailing whom, but instead just how many letters someone is receiving.

[anigbrowl]

Yes, and for most private mail that's perfectly fine as long as there is adequate postage. People who are required to furnish an accurate return address include senders of prepaid mail (you know, where it's printed on the envelope with a number), distributers of periodicals, senders of priority and registered mail, and a few other similar examples.

[nitrogen]

It's still possible to identify where the letter was sent based on the postmark and unique barcode printed on the envelope when it is scanned.

Based on https://news.ycombinator.com/item?id=5986635 and https://news.ycombinator.com/item?id=5986011

Edit: this comment is for scrabble's sake, as you and I already exchanged comments on the subject.