@evan
"Key concepts in IP multicast include an IP multicast group address,[2] a multicast distribution tree, and receiver-driven tree creation.[3]"

My impression is that the setup time for this is justified with streaming (a live video or audio multicast) but my gut is telling me it wouldn't be efficient for per-message attachment distribution.

Then I think you use it not per message but long-term server-to-server message distribution groups, but don't understand how you handle retransmission