Mailspring is an easy-to-use cross-platform IMAP email client.
It uses a new C++ sync engine based on Mailcore2 and utilizes roughly half the RAM and CPU of other email clients. It idles with almost zero "CPU Wakes," translating to great battery life when used on a laptop.
Mailspring is built on the modern web with Electron, React, and Flux. It is designed to be extensible, allowing you to create new experiences and workflows around email. Mailspring comes packed with useful features like a Unified Inbox, Snooze, Send Later, Mail Rules, Templates, and more. There is a Pro version (monthly subscription), which adds more features for users who send numerous emails like link tracking, mailbox analytics, read receipts, contact and company profiles, etc. All of these features run on the client, and it will not send your email credentials to the cloud.