- Major redesign of the user interface
- Changed periodic background sync and push implementations to work much more reliably
- Deprecated support for the WebDAV protocol; new accounts can no longer be added
- K-9 Mail now requires Android 5.0 and newer
- Added support for Autocrypt setup message
- Added support for encrypted subjects
- Don't use default signature when setting up a new account
- Allow installation on external media (Android 6+ with adopted storage)
- Removed keyboard shortcuts for menu items; on some devices they conflicted with system shortcuts
- Removed the broken option to store the message database on external storage
- Removed the 'remote control' interface third-party apps could use to change some settings in K-9 Mail
- A lot of other bug fixes, internal changes, and improvements
- Check out the changelog entries for 5.7xx versions for a lot more details
The forum thread for this release: https://forum.k9mail.app/t/k-9-mail-is-back-5-800-release/1164