Howto downgrade any app to the previous version without losing data on android oreo?
What you want is not possible and for a good reason: updated data structures (files, database, etc) are often incompatible with old app versions. This is why you can downgrade debuggable packages (for development purposes) only.

If you have a problem with FairEmail, please report it in this forum:

https://forum.xda-developers.com/android/apps-games/source-email-t3824168