There is no way to be 100% sure the email is vaild.
What you can try to do is connect to the mail server, start a mail session and "ask" the server to accept mail for the given address.
Some mail servers will return an error if the address is not listed, but others will accept any address, as long as it is in the server's scope.
As for the site you've mentioned, try to check this address for instance: not_a_valid_address@microsoft.com
You will get an answer that the server accepted this address, but it doesn't mean it's a valid one...