OK, I was being a bit thick there, to look up a email address you presumably simply hash it (presumably with some kind of canonicalization) and use that as the key for your lookup for a matching +end.
I'm not sure, but this is what i think i get out of this. You really don't need an email, this could be a hash or whatever. i think of more apps or services use some protocol like this, there should be a unified why to "lookup" for an account or something.
For example, if you have a decentralized social network that is based on the same software, say "facespace", every of those software nodes would lookup for the same identifier. i guess...
