From Business Software Products:
MailList Validator is designed to validate e-mail addresses and see whether or not certain addresses exist. The program's main objective is finding invalid e-mail addresses. This will save a great deal of your time when doing mailings and minimize the number of bounced messages from non-existing addresses. Mail List Validator uses Rules for determining bad e-mail addresses. That allows to achieve very high accuracy of the validation. The program is capable of finding all invalid addresses in a list, and it is absolutely free from the false trigger instances where a valid address would be marked as invalid. The program supports a special validation mode where it only issues a DNS request to resolve MX servers for the e-mail address' domain name. Using this mode, you can quickly find and remove addresses with domain names that no longer exist.