The best Encrypted Chat alternative to Thunderbird is K-9 Mail, which is both free and Open Source. If that doesn't work for you, our users have ranked more than 50 alternatives to Thunderbird, but unfortunately only two of them are Encrypted Chat Apps.
K-9 is an open source email client for Android with multi-folder sync, email signatures, Bcc-to-self, return-address configuration, keyboard shortcuts, Exchange Support, message flagging, IMAP deletes, saving attachments, configurable notifications and more.
Transform your email inbox into a collaborative workspace with Spike. Enjoy integrated chat-like emails, tasks, collaborative notes, and group projects. Seamlessly manage multiple accounts, organize priorities, and schedule meetings across devices.
Israel
Keeps the style of working with email as it should be, like we are used to from proper desktop Mail clients. It's unlike modern fancy clients with couple of buttons without even providing an option to view headers.