What are some alternatives?
When comparing mails.wtf and Email Verifier, you can also consider the following products
Hunter - Find all the email addresses related to a domain
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
LeadGibbon - Access millions of business contacts, with verified email addresses and phone numbers
NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Mail Dump - Powerful Extensions for email addresses extracting, verifying and finding
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.