One important thing K-9 does that this doesn’t: realese on F-Droid.
What is this? At least provide a repository like DivestOS… while you are at it, get the code for the free software off of proprietary Microsoft GitHub.
How on Earth do you lose this support in the handover shuffle—especially knowing the audience a third-party email client? I would say it shouldn’t be released if F-Droid support isn’t there since it isn’t something you would want to back-burner.
I'm pretty sure it's an issue on F-Droid's end, as it's always a few days behind for all my other apps that get released on the play store as well. IIRC they have a release process that involves them compiling the packages that takes a while to run.