Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
ZoomInfo - ZoomInfo is a B2B database providing detailed business information on people and companies.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Clearbit - Clearbit provides Business Intelligence APIs
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
InsideView - InsideView aggregates and curates all the company and contact data, news and social insights, and professional connections you need to do business better.