Thunderbird — #5 Alternative to Gmail / Outlook
Rejuvenated open-source desktop and Android email, calendar, and OpenPGP client
With its Supernova/Nebula redesign and Thunderbird for Android (built from K-9 Mail), Thunderbird unifies multiple IMAP, JMAP, and Exchange accounts locally with native OpenPGP encryption and offline search.
License: MPL-2.0 · Pricing: Free & Open Source · Compare all Gmail / Outlook alternatives · Official Website