What are some alternatives?
When comparing Thunderbird and Mailspring, you can also consider the following products
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Postbox - The Power Email App for busy professionals, like you.
eM Client - eM Client is a Windows-based email client for sending and receiving emails, and managing calendars, contacts and tasks.
Apple Mail - There’s big news attached.
Claws Mail - Claws Mail is an email client (and news reader), based on GTK+, featuring Quick response