It doesn’t look like that app is under active development. According to Google Play store, that app has not been updated since 3 April 2018, and the source code hasn’t been updated since 29 Jul 2017.
So don’t expect it to be kept up to date or to get any of the bug fixes that have gone into K-9 since the version from which that fork was created
You’d probably be better off sticking with v5.600. As the creator of Open Mail says in his README.md
Its mine personal attempt to make a Android OpenSource Email App with different Name, Logo & Icon to relate it with email. Its still Beta from my side as its some basic changes to orginal code of K-9. Would try to find time to add more into this app. This is noway to replace orginal K-9 Mail app or compete with them.