Hi,
Welcome to JavaRanch!
The SMTP standard includes a "VRFY" command which you used to be able to use to verify whether an email address was valid at a particular host. Nowadays, virtually all sites implement this as a no-op, so it's pretty much useless.
The only way that works is to send an email to the address containing a unique URL. The email should instruct the recipient to visit the URL. A
servlet at the URL can then record that the corresponding email is valid.