What are some alternatives?
When comparing mails.wtf and LeadGibbon, you can also consider the following products
Hunter - Find all the email addresses related to a domain
Mail Dump - Powerful Extensions for email addresses extracting, verifying and finding
Norbert - Norbert is an online tool that helps you to find anyone’s corporate email address.
Snovio - With Snovio you can find emails, verify them and run your email campaigns within minutes.
AeroLeads - AeroLeads is a lead generation software which finds prospects with email and phone number.
Find That Email - The yellow pages of emails.