WebPlus X6: Verify email address

I have a contact form into which the user enters his email address and telephone number (amongst other things). Recently there have been a couple of cases where the user has entered his email address incorrectly and we had to contact them by telephone, but I'm concerned about the possibility (albeit probably a small one) where the user makes a mistake in both his email address and his phone number (or am I being a little bit paranoid?).

I'm already validating the email address for correct format etc both clientside and serverside, but I would like to also check that it really is a valid email address which actually belongs to somebody and has been typed in without mistake by the user. I don't like asking the user to enter his email address twice because I think that it annoys a lot of users, so I'm thinking about using an online email verification service. It needs to be "real time verification" of course, so that I can advise the user of his error without it unduly holding up my response to his form submission. Does anyone have any experience os these things?

I've been looking at http://verify-email.org/ which seems to fit the bill reasonably well and which does seem to actually work. I don't want to use their free service because it is ad supported, but they do offer an "ad free" service for a very reasonable price. Does anyone have any experience of this particular service, or maybe perhaps recommend something else?

Alternatively, how difficult would it be to write such a facility in Perl (PHP at a push, but I'd sooner use Perl). Is it possible to write such a beast in Perl?

Mike