Unfortunately, it's not very easy to check email validity directly with
an SMTP server. There are two main reasons:
1) Quite often SMTP servers are behind firewalls that block port 25,
so they are not reachable. Also, outgoing port 25 is commonly blocked
as well.
2) Many SMTP servers will not report an email address as invalid because
the mailbox is setup with a "catch-all" address.
The best way to manage valid/invalid email addresses is by fielding bounced
emails. The BounceAddress property can be used to have bounces sent
to a separate address where bounces can be processed by a program.
(One possibility is to use the Chilkat Bounce component...)