Hey, if you're looking to test whether or not an email address is syntactically valid, I think I've stumbled across just the tool for that. Check out the very cool isemail.info by Dominic Sayers.
This doesn't connect to a remote server nor does it try to determine if the email address is actually deliverable. It tells you if there are too many dots in the wrong places or otherwise contains something that isn't allowed in an email address.
If an address fails the validation process, it tells you exactly why that address failed; what is in it that is disallowed, and why is that disallowed. Very, very kind of Dominic to have taken and distilled the RFCs down to something you can actively test against.
(Of course there are some email addresses that violate RFC specs but are still deliverable and otherwise "valid." But that's a problem for a different day.)