There is a high level of availability of solutions that will help you discover prospects' emails, but not many have the list-building functionality. Their pricing is affordable, and this product was designed by Email-Experts.
Search whichever particular podcast application you use for Ryan O'Donnell and find "The Brutal Truth about Sales and Selling" hosted by Brian Burns, and listen to Ryan talk about his story and why he developed this product.
Then I highly suggest creating an account. They give you 10 free email verifications/month, but I'd strongly consider upgrading to access the list building features. Review collected by and hosted on G2.com.