I've recently noticed a long string of "Email Verification" websites surfacing.  The one I'm choosing to make an example of is "Verify-Email.org"  and for good reason. 

If you look at the small print at the bottom of the page at the copy right, you'll notice a link for "Email Software".  When you visit that link, it takes you to the companies website, email-unlimited.com.  The company itself is a bulk email/marketing company, and if you put 2 and 2 together, you get a spam trap! 

I don't advise using any of these sites, they're all the same.  If you'd like to wax intellectual on the logistics of why they can't possibly work on any legal level, let me know.  The truth of the matter is that these companies who actually charge for this service...  are charging you to build bulk mail lists for them and sending letting you know if the domain is valid or not.  Most companies have protection up to defer it's query so unless you plan on checking the emails of smaller private websites, who don't have the where-with-all to install security, they're useless and stealing your emails =p 


cheers^^

ps: lifehacker even had an article up about the site which was removed due to the discovery of this particular "Email Verfication" website as being a spam trap. 
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Edited:  It turns out it actually works with some sites and not with others, but it's completely independent to the mailing system.  The problem here is that it checks first against "MX Records" and then if there are none, then by domain validity. 

The MX record in layman's terms is simply a map provided by the domain "xyz.com" to the server hosting that specific mailbox on it.  If these records exist, than you may or may not have an accurate representation of whether the email does in fact exist.  The reason this is questionable is because if the MX record has orders to route any "email validation requests" or emails to a specific server designed to handle spam content, then it would return as being valid by default.  Any "email validation request" will return as being existent b/c it's sending all unknown requests to the server, and the site sees that the request has been sent to a server, hence the return is "Valid". 

Most large firms and companies have similar spam filters in tact to prevent email abuse before it becomes an individual/per user account problem.  There are several other methods for hiding email validity that these companies use to protect their employees, servers, and general interests. 

In short, will this email checker work...  you'll never know 100%.  It will vary company by company, by security administrator, by need for deterrents.