And if you use a remailer to send a message to a distributed PIR that message recipients poll and send fixed size cover traffic to in fixed duration cycles (although accumulating, so real time is not a requirement) , then it becomes all but impossible to correlate traffic from sender to recipient. The attacker would need to own n out of x PIR servers to narrow in on the message recipient past the total anonymity set size of the PIR cluster. And theoretically x and n can always grow larger. And of course there is the mixing during transit as well, which additionally makes it difficult to even correlate traffic at any other point in its transit, in addition to making the message sender extremely difficult to trace even for the messages final recipients. This is the current state of the art for significantly scalable anonymous communications. Of course the true state of the art for anonymous communications is a dining cryptographer net (DC-net) as it is mathematically proven as always maintaining anonymity for each user to the total set size of users, but it is actually very limited in other ways and I would compare it to the one time pad of the anonymity scene.