I heard this from one of my friends last month talking about encryption and decryption capabilities apps.
I think this is similar to Openkeychain, you can find it on the Google Play Store.
It's K-9 mail, once installed, you can integrate it with OpenKeychain.
It only supports Android OS only it does not support the iPhone IOS version yet and found their open-source code
here.
It is a thunderbird from Mozilla and their Android version is K9-mail there are lots of bad reviews about this k9-mail on Playstore but according to them, it's the UI they want the old version compared to the new one.