Sending a verify link to the registrant to verify is the only way to be sure an email address is valid (at least for a short period of time).

On Mon, Apr 7, 2014 at 4:59 PM, Chris Miller <cjm@tryx.org> wrote:

From: "Walt Daniels" <wdlists@gmail.com>
To: support@drupal.org
Sent: Monday, April 7, 2014 1:55:59 PM

Subject: Re: [support] Many false applications for accounts

Yes you can prob mail providers and determine if the email is valid, but they may lie. I believe AOL replies yes to any request, at least they used to.

Hi Folks,

Isn't the traditional way to handle this simply send a link to the registrant's email account? Bogus e-mail; no access.

Chris.