You can't really confirm it.
You can of course check if there is a MX entry for the domain, so you know that it may accept smtp.
There are a number of web-based services on the web which claim to verify email addresses, but basically they can only verify:
Valid domain: "email@example.com" is not valid
Valid user: verify if the user and mailbox really exist
It may also be possible to do it yourself (from within FileMaker) if you understand enough about how to "talk" to a mail server. (I don't.)
Seems like you're asking several questions...
Can you tell if an e-mail address is of a valid format? (ie not firstname.lastname@example.org nor www.time.com)
Can you tell if an e-mail address actually exists?
Can you tell if an e-mail has been delivered?
The answer to all three is no.
You can get close with #1, but dive into Wikipedia's entry on e-mail addresses and you'll see the complications. #2 and #3 rely on things far beyond the scope of a FileMaker database.