Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
MJML - The open source framework for responsive emails
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Foundation for Emails 2 - Create responsive HTML emails that work on any device
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.
HTMLEmail.io - Responsive HTML email templates for startups & developers