Mailspring is described as 'Boost your productivity and send better email with Mailspring, a beautiful, fast email client for Mac, Windows, and Linux. Mailspring is free to use and an optional pro version adds read receipts, send later, reminders and more' and is a very popular Email Client in the office & productivity category. There are more than 25 alternatives to Mailspring for a variety of platforms, including Windows, Mac, Android, Linux and Web-based apps. The best Mailspring alternative is Thunderbird, which is both free and Open Source. Other great apps like Mailspring are K-9 Mail, Betterbird, Evolution and FairEmail.
Thunderbird is free, stores nothing in a cloud service, so you control your email and the storage of your personal data. It supports encryption (natively, now) and with plugins is able to work with GMail and Exchange accounts without giving all your info to those behemothim. Easily customized mail rules allow you to route mail easily where you want it, and even without plugins it can easily protect against unwanted attachments, etc.