NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
Clearbit - Clearbit provides Business Intelligence APIs
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
ZoomInfo - ZoomInfo is a B2B database providing detailed business information on people and companies.
DeBounce - Email Validation, Email Checker, Data Enrichment and Appending Tool. Using DeBounce remove invalid, disposable, spam-trap, syntax and deactivated emails.
InsideView - InsideView aggregates and curates all the company and contact data, news and social insights, and professional connections you need to do business better.