A complete centralized identity manager would have in the same database with your password your email address and all other personal information typical websites might want about you. With your permission, the web site could get that whenever it wants it, thus saving you the trouble of filling in forms to register with each web site, and then updating them all when the information changes.
A mail forwarding service might be useful, but I don't think it adds much to the server just giving the sender the email address.
I would love it if I could use a web site for the first time (and every time) by just saying who I am (with some short string such as an OpenID URL).
And if that doesn't tell the web site enough about me, it can tell me to come back when I've added the required information to my identity manager and released it.
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