tldr: each stamp has a unique code. I didn't really believe it until I hit the single mention of "unique" in the text.
since all mail will now have a unique tracker on it, all the issuer need do is log who purchases which stamps and you can now track who sends mail.
this could have a chilling effect on many areas of society who communicate via items or letters sent in the post, as the anonymity and neutrality of mail is eroded.