Remotely accessing an IP address inside a target computer is a search
washingtonpost.com
washingtonpost.com
"Is there a limit on how many different places can be searched under a single warrant while still satisfying the requirement that the warrant describe the “place to be searched”? Can a single warrant justify a search of thousands or even (hypothetically) millions of computers, all used by different people who don’t know each other? At what point does the use of a single warrant to search many places make the warrant a general warrant that the Fourth Amendment prohibits?"
It also seems to miss the point. If a person has a hidden identity and a stalker breakers into the post officer in order to obtain the protected address, is the stalker committing a crime towards that person or can they just claim that addresses "is public" because the sender "voluntarily shared the information". The fact that the sender has protected identity should give a strong hint towards their intentions, even if the knowingly used the postal service.
If the FBI used a sybil attack and only passively gleaned the information as the "letter" came past, rather than do a breaking and entering, would it change anything? We could still distill this legal issue as intentional protected information being knowingly intercepted without a warrant. It would be a interesting case, but first the court need to decide if planting malware on citizens computers need a warrant or not.