A "completing" function as in Ludwig would maybe go fine, it automatically searches while typing in the item name and displays the items that containing the word.
Maybe a OnClick dewdrop that pops up when clicking in the recipient text box?
But i don't know how much load such a thing would cause if you got >100 entries in your address book.
otherwise a lod module which does the same but only gets loaded when ppl click on a button near the textbox so performance fetishes don't get poked by stuff they don't need :D
just some ideas, may they help :)