Clearbit - Clearbit provides Business Intelligence APIs
Hunter - Find all the email addresses related to a domain
ZoomInfo - ZoomInfo is a B2B database providing detailed business information on people and companies.
Icypeas - This email finder has the capacity to bypass catch-alls
InsideView - InsideView aggregates and curates all the company and contact data, news and social insights, and professional connections you need to do business better.
Apollo.io - Apollo’s predictive prospecting, sales engagement, and actionable analytics help the teams to reach its full revenue potential.