I see specialized servers to handle high volume messages not that different than ATP. That protocol is designed to scale up from intra-domain server(s) to external agents. The selection of a specific implementation then becomes a load and scaling issue.
The preponderance of messaging, whether between avatars in the form of text chat, or between scripts (attached to avatar or otherwise), is going to be mostly intra-domain; that is, it is architecturally local traffic. In HF, avatar to avatar chat is for practical purposes script to script coms.
We see that already in many other virtual worlds. And, that messaging is usually not long-term stored, just short-term shallow buffers to permit flow control. That’s the kind messaging that the message mixer foundation could handle well.
I agree that store and forward messaging (classic instant messaging that works when the recipient is not around or is at a remote location in which its presence cannot be determine immediately) is best served by a store and forward system.