[deleted] I am using FairEmail at the moment, though I used K-9 Mail on another phone in the past. I prefer the former for its lightweight footprint, its extended customization options and the fact that it seemed updated more regularly than the latter. The only thing I do not like, so far, is its UI; I remember K-9 Mail doing a much better job at that.
Overall, it is a solid email client, with sane defaults. I have not tested OpenPGP with it though.