Delta Chat

Free open source end-to-end encrypted messaging system that works through email. This means if you both have Delta apps running you can chat in encrypted mode, but even people without Delta can chat with you through their normal email account. It’s available for Android, iOS, Linux, Mac and Windows.

Fediverse address:
@delta@chaos.social

K-9 Mail

Very popular free open source e-mail app for Android. It is due to become the official Thunderbird app for Android devices.

Fediverse address:
@k9mail@fosstodon.org

Tuta (formerly Tutanota)

Independent privacy-friendly email and calendar provider which is ad-free and paid for entirely by their users’ subscriptions. There are free open source apps for Android, iOS, Linux, Mac and Windows. There’s also support for E2EE and 2FA wherever possible.

Fediverse address:
@Tutanota@mastodon.social