This will return a boolean (true/false) if the email address is valid or not.

Rapleaf's Address Book API makes it easy for users to import webmail contacts from Gmail, AOL, Hotmail, and Yahoo. You can access names and email addresses of the person's contacts, and invite them to your site, import social relationships, share content, and more.

Rapleaf maintains an up to date code on the importer, and never stores passwords.