From time to time people ask us if the ISN can help determine if an email address is correct. Candidly, we are right there with you, we sure wish it could! It would save us all so much time!
An Email Address is only valid if the Postmaster says it is. The Postmaster is the computer in control of delivering the system. When email was developed it was designed much like a traditional snail mail system. A mailed envelope (snail mail) is only proven to have an invalid address if a delivery could not be made. Email was developed using this same methodology. Thus email still requires a delivery of the message first. The Postmaster determines if email address is valid and then either (a) delivers the message to the recipient or (b) creates a new message and sends it back to you states a delivery failed.
Here's the challenge, the Postmaster will not talk to anyone he/she only receives email - that is all he/she knows how to do. Therefore there is no way to check to see if that address is even valid on that system. (blah!!)
Here is an example of an email going to firstname.lastname@example.org. The email arrives and the Postmaster takes the email. He/she then looks at all of the email addresses, aliases (etc) and checks to see if he/she can deliver the mail. In this case the Postmaster would not find the address and send you a mail back stating it could not be found.
Up until recently there hasn't been a consistent resource for validating emails and there isn't a tool we can integrate into ISN at the moment.
At ISN Support we check emails using Email Hippo