Is there an existing protocol where a sender does not know who he is sending data to?
If you want strong protections for the identity/anonymity of the participants, you might be interested in systems for anonymous communication, such as Tor.

If you're just interested in systems where the destination's address isn't necessarily known under normal operating conditions (but it's not a critical security goal to ensure it can never be known), you might be interested in peer-to-peer overlay networks, such as Chord and many others.