Nice little gimmick, thanks! We put it in our sign-up script as well. Since most of our clients are Dutch, we added the most popular Dutch e-mail providers (+ a big German + Belgian):

casema.nl, chello.nl, hetnet.nl, home.nl, kpnmail.nl, kpnplanet.nl, live.nl, online.nl, planet.nl, quicknet.nl, schuttelaar.nl, skynet.be, t-online.de, tiscali.nl, upcmail.nl, wanadoo.nl, wxs.nl, xs4all.nl, zeelandnet.nl, ziggo.nl, zonnet.nl.