Recipient Anonymity

[Ulf Möller]

>Except for the additional server-to-server communications necessary to
>broadcast new messages into the system, the bandwidth utilization is
>comparable to sender-anonymous remailers; it scales linearly as the
>number of parties involved in the delivery of a particular message.

How are messages selected?  It seems that the recipient needs to know
the message IDs of all messages in the system, and must have a way to
identify his messages.