A little late in replying, but the fact I had never heard of K-9 until this HN post kinda proves my point about name recognition. Last year I actually tried 5 or more different Android clients. I didn't try K-9, if it I even saw it. I must have judged it as untrustworthy or not worth trying for some reason.
https://f-droid.org/app/com.fsck.k9