Works with K-9
Many other Android applications build on K-9’s functionality.
Searching for K-9 on Google Play will turn up lots of things;
here are some of the more useful-looking ones.
Security / Encryption:
- OpenKeychain - End-to-end email encryption with the OpenPGP standard
- X509Tools - Allows S/MIME based email encryption in K-9. Note that this application is no longer supported by it’s developers so may have serious unfixed vulnerabilities (Last release September 2013)