What are some alternatives?
When comparing mails.wtf and NoBounce Email List Cleaner, you can also consider the following products
Hunter - Find all the email addresses related to a domain
Email Verifier - Email verifier app lets you verify email.
LeadGibbon - Access millions of business contacts, with verified email addresses and phone numbers
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
Mail Dump - Powerful Extensions for email addresses extracting, verifying and finding
useBouncer - The most intelligent email checker, that lets you focus on valuable relationships! Verify email address with simple App or easy-to-integrate API.