I came across Clearout.io by chance and found out it to be the most cost-effective & user-friendly interface to use! I compared the effectiveness of the email verification with other tools in market and surely I could find the difference in quality.
Clearout + GoConvert
Use GoConvert and Clearout to maximize the ROI of your sales and marketing spending. Enrich information in your Clearout account, improve contact and deal information, and start visualizing marketing and sales performance. GoConvert works around the clock to automatically fill in missing data. Your insights get better all the time.