What are some alternatives?
When comparing mails.wtf and useBouncer, you can also consider the following products
Hunter - Find all the email addresses related to a domain
NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
LeadGibbon - Access millions of business contacts, with verified email addresses and phone numbers
Email Verifier - Email verifier app lets you verify email.
Mail Dump - Powerful Extensions for email addresses extracting, verifying and finding
NoBounce Email List Cleaner - Email list validation & cleaning maximizes the efficiency of your email marketing campaigns and protects your email sender reputation.