Back in 2001, a 94 year old woman died after being exposed to weaponized anthrax.
At the time, the government let out a little secret if you carefully read the accounts. Investigators discovered that the letter to the woman in Connecticut passed through the same sorting machine a few pieces after the letter sent to Congress. If that wasn’t already known to you, the USPS has long tracked every single piece of mail, not just persons suspected of criminal activity. The outside of an envelope has no expectation of privacy.
The USPS now proudly declares that they take and retain a photograph of every envelope they deliver (unless the postal workers deliberately discard the mail)