Private letters have long been acknowledged as covered by the fourth amendment, that is that searching requires at least a warrant based on probable cause. It’s changed over times whether searches of papers are per se unreasonable [1]. Consent can authorize agents to open mail of course, but it might be an interesting question whether it’s an unconstitutional condition to require waiver in order to access the mail services.

[1] https://www.repository.law.indiana.edu/cgi/viewcontent.cgi?r...

Seems like such easy ground for 4th amendment creep, I'm surprised it hasn't been tried yet.

Get all the big companies using it, then before you know it REITs and management companies will start using it for multi-tenant residences. Boom; 3 letter agencies will start popping up arguing there's no reasonable expectation of privacy since the user agreed to a "CommunitySafe mail protection services" clause buried in the lease.