One or two years ago Mozilla hired the main developer of K-9 Mail. The dev previously raised enough donations to work on K-9 Mail for a while and modernize it after not having an official release for years.
Not tried the app version. Been using Fairemail for a while now, since k9 was unmaintained.
Fairemail is well maintained. Quick. Supports multiple accounts very well. Loads of features (could be a downside for those who like things simple). Designed with security and privacy as top priorities right from the start. Open source development. For a long time its been the best email client on Android IMHO.
I think I've been using K-9 Mail for 10+ years or something. The settings were kind of all over the place but it has always been one of the email clients with the most features.
K-9 is being used as the base for Thunderbird for Android but it isn't there yet. FairEmail is a lot closer in functionality at the moment.
I use Thunderbird for Gmail (among other accounts) and it has to regularly compress my Gmail box, which none of the Android clients do - in my experience, Aqua Mail, K-9 and FairEmail all struggle with a decent sized Gmail mailbox after a while.
For me it's good, let's me leave emails on the server so my desktop can read them too. Let's me reply below or above and compose in ascii. Doesn't impose ways of working I don't want.
I do not, but only because I use ProtonMail and don't have premium so I don't have IMAP/SMTP bridging.
I used it all the time back around 2011/2012 and it was pretty great, but that was admittedly a long time ago so I imagine lots have changed since them.
I do have protonmail premium with my own domain, but the IMAP bridging is for desktop. It's a small program that kind of works as a MITM, one side connecting to protonmail securely using, i guess, not IMAP, and the other acting as an IMAP server that your mail clients connect to. On mobile you have to use the protonmail app, AFAIK.
Used to. Prefer Aquamail. I'm a thunderbird user on my workstation though. The latest changes were controversial, but it's fine once you enable the system title bar and hide the menu bar.
Yes, I use K-9 for my outlook/365 accounts and it works fine. I also have my NAVER account there but I can't send emails because I've been too lazy to configure the SMTP settings properly.
It requires 2-step verification which requires a phone number, security key or google prompt for google accounts. Hard pass :D
edit: i stand corrected, this seems to have been changed