What is a good FOSS email app

Fairemail is currently the top for foss email on Android. K9 may take that spot now that there's someone actively working on it with goals in mind. Thunderbird is a really solid client, so if they put similar time into k9 (but please, leave the icon lol), it may end up blowing everything else away, even the non-FOSS options.

K-9 seems promising, it is acquired by thunderbird and planning to become thunderbird bird mobile. It has office 365 support, but IIRC it is pretty convoluted to add a office 365 account. https://www.f-droid.org/en/packages/com.fsck.k9/
Another alternative is fairmail: https://www.f-droid.org/en/packages/eu.faircode.email/ , developed by the developer of netguard.
- I used to use K9 long time ago, but had issues with notifications not working properly. I've been using something not FOSS which was OK but I wasn't 100% happy with. After reading about FairEmail on another thread the other day, I've now adopted that and so far it is working great. It has way too many options though!
  
  I generally dont want notifs on the email anyway ! I have the official gmail app on my android and i dont get them notifications
- Ah i trust thunderbird !
- k-9 is good for storing gmail accounts ig

FairEmail is also recommended by many. It has even more functions, but is therefore also a bit more complicated.

for Android: k9-Mail

Fairmail on Android, Thunderbird elsewhere. KMail if you reall, really want to go through a painful learning process.
- Sorry, maybe I am too deep in the software jungle but what is the painful learning process? No front, I am genuinely curious.
  I setup k9 a few years back and thought it comprehensible. The only thing I remember a bit annoying was migrating GPG keys over.
  
  KMail does some things fundamentally different than you'd expect and in typical KDE fashion you can configure everything and then some.
  For example there is no strong coupling between accounts/addresses/identities. You can send from any account with any identity. You always have to make sure you chose the right one. By default, all your mail will go over one server, whichever you registered first I think. There was no way of setting a specific one for each identity, again you have to choose the right one every time. This is obviously inappropriate for some setups, e.g. if you have private and work-related accounts.
  I think the way Thunderbird handles things is much more sane, because there is only exactly one identity for each account and they are all kept separate. It's ok for KMail to offer more flexibility, but the default behavior is a huge mess and lacks some functionality imo
- I use this exact combo too. I heard k-9 mail will become thunderbird on android, do you think this will make it any better or shall I just stick to fairemail
  
  I'll just stick to Fairmail for now, I don't know if k-9 can keep up feature wise, but I kinda doubt it.

I can recommend https://email.faircode.eu/.
- For some reason I was under the impression that development had stopped for this app.
  
  There are regular commits on the repo https://github.com/M66B/FairEmail/commits/master
  
  This is the app that receives the most updates for me, a few years ago and still true to this day (there's a new release for every commit or something lol). They are in for the long run it seems.

Using K-9 and quite liking it, otherwise just browse F-droid everything there is Foss.
- Yeah i have fdroid already and have replaced 90 percent of my apps with f droid apps !

K9 is good.

I use fairemail
- Thank you for the recommendation, just downloaded it and this looks amazing!
  It's incredible how long it has taken me to find something with customizable notification actions and sufficient options! This is the first time I've ever been able to have new email notifications with both Archive and Spam options.