The USPS can&#x27;t read the letter without breaking the seal. That&#x27;s different from Tor, where the three parties can team up and exchange data, and they&#x27;ll know who visited what, without breaking any seal.<p>If the mail is unsealed, they&#x27;re allowed to open it. See <a href="https:&#x2F;&#x2F;www.law.cornell.edu&#x2F;cfr&#x2F;text&#x2F;39&#x2F;233.3" rel="nofollow">https:&#x2F;&#x2F;www.law.cornell.edu&#x2F;cfr&#x2F;text&#x2F;39&#x2F;233.3</a>