Personally I have been using Kleanmail for the last 2 years and I must say it's one of the most trusted, powerful and reliable and secure email validation tool with a very proactive and responsive support team. What makes this tool standout among its competitors is a cloud based email validation tool that helps us to cleanse our customers database. I also like how this tool integrates with our financial department to make sure our emails database are secure and clean with its capabilities to differentiate the valid and invalid emails IDs and assure that emails are sent only to the right contacts. I also like it saves our domain/IP from being blacklisted by most email service providers, hence increases more chances of our promotional emails being delivered to our customers inbox at the right moment.