Jon nails it as usual. Mail Channels, sxip et al are doing interesting things by taking advantage of the DNS layer.

From Jon Udell: The architecture of intermediation.:

QUOTE

In the web services realm, buzzwords like "concern-based" and "aspect-oriented" point the way forward. Privacy is a concern that I may have with respect to my use of del.icio.us, and a potential aspect of my transactions with the service. Joshua Schacter shouldn't have to modify del.icio.us, and Steve Mallett shouldn't have to fork it, in order to satisfy this need. A lightweight intermediary ought to suffice. The popular style of intermediation may or may not wind up being WS-*, but something like it will be how we avert these kinds of conflicts and tailor services to our needs.

UNQUOTE