Mailspring

Mailspring indexes your mail so you can use advanced, Gmail-style search queries and search message contents across all your connected accounts. Never lose an email again. Your signature should leave a lasting impression. Mailspring's beautiful signature editor makes it easy to create and use custom signatures, even with images.
Using a single inbox for all of your email accounts helps you get more done in less time. Mailspring 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. Business happens in every language. Mailspring can translate messages written in English into Spanish, Russian, simplified Chinese, French, and German--right inside your draft. Mailspring auto-detects the language you're using and spell checks for that language. No need to change your settings.