Mailspring - A beautiful, fast Webmail
Mailspring is built on the modern web with Electron, React, and Flux. It is designed to be extensible, so it's easy to create new experiences and workflows around email.
Mailspring comes packed with powerful features like Unified Inbox, Snooze, Send Later, Mail Rules, Templates and more. Mailspring Pro, which you can unlock with a monthly subscription, adds even more features for people who send a ton of email: link tracking, read receipts, mailbox analytics, contact and company profiles. All of these features run in the client - Mailspring does not send your email credentials to the cloud.
You can have unified inbox as it supports every major mail provider—Gmail, iCloud, Office 365, Outlook.com, Yahoo!, and IMAP/SMTP—so you have a single, streamlined command center for all your messages.