Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
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.
MailClark - The Slack bot for external communications