Email addresses are pretty easy to validate; the rules are all specified pretty clearly, and there are a large number of regexes for checking them:
Google "Regex email PHP"[
^] will find you loads.
But...that doesn't mean it's a "real" email address, or that anyone reads what is sent to it if it is.
The only way to check is to send an email which requires a response, and make sure you get it. And that means that the email address was valid when you sent the email, no more - there are web sites out there offering email ID's that last ten minutes, for example!