autocomplete in Hotmail (address To), how it is working?

I specially has used hotmail and I like its autocom开发者_StackOverflow中文版plete for To Address. It works very nice like it not only shows saved contacts but also suggests address on which I have sent email but they are not saved in contacts.

The most important thing is it is very fast. It is not ajaxtoolkit's automcplete what is this control. Plz guide me about its implementation I have to do same in a web.

thanks Thanks THANKS

Looks like they are not doing any ajax. When I checked it through firebug I could not see any network call while typing in To box. I think they are downloading entire address book to client side at startup, thus reducing any further ajax call for To box. Didn't dissect it further.