Smart Email Verifier allows you to choose the level of verification you need.
The Syntax verification will make sure that the email address is correctly formatted. The Domain verification certifies that there is a mail server available for email to be sent to. A SMTP verification will send a 'verify' command to the mail server to make sure the email address is deliverable. To use the smtp verify, you will need to have port 25 open on your network. You may need to contact your ISP to open this port for you.