Mailspring

Mailspring is a fast, beautiful desktop email app for Linux. It features a unified inbox, powerful thread-based email management and beautiful themes, as well as standard features like mail rules, templates, and more. It uses a C++ sync engine that idles quietly with a small memory footprint, and features rich integration with Linux desktop environments (notifications, system tray indicator, etc).

Mailspring is open-source, and creating an account (a "Mailspring ID") is fully optional. Signing in with a Mailspring ID enables features like link tracking, read receipts, and rich social profiles alongside conversations, and a paid "Pro" version is available for professional users that make heavy use of these server-connected features.

Email sync is fully performed on your machine. Mailspring does not send your email credentials to the cloud, perform Gmail / O365 Oauth through it's servers, or transfer your email data off your machine.