K9 with the old interface, the original one.

I picked K9 among the other clients because of that interface so I was in that 50% of users that were pissed off by the new UI. Luckily email protocols did not change and I can still sideload the APK on all my devices.

I have used Aquamail https://www.aqua-mail.com/android since the mid 2009 on my android devices. It's been solid and it is super customizable. I think the only downside is they changed their payment model a while back and if you didn't buy it back then it costs monthly fee or a $99.99 for a lifetime license. I highly recomend it if you do a lot of email on your phone.

I use k9/thunderbird for private mail. But for my org's on prem exchange account I use nine folders, as I also want to sync calendar and contacts. (We have a davmail gateway but it has quirks)

I use ProfiMail on Android. Loving the app since Symbian era. K9Mail is ok too.

Isnt K-9 Mail supposed to join forces with Thunderbird? K-9 Mail works fine for me.

I use FairEmail in Android.