If you're trying to verify users e-mail addresses on your machine then don't do this at all--check your local user database instead. If you don't have one, make one. (And odds are you do have one, you just need to find it. What, where, and how depends on your system)

(If you're looking to verify addresses on other machines in realtime, you can't. Stop trying, it's impossible in general and all you'll do is make a mess of things, probably leave a spam and/or abuse hole, and piss people off)