FairEmail and K9 are the only real options on mobile that I know of if you want open source. Comparing the two, I'm much happier on FairEmail though K9 wasn't bad either.
Particularly the link dialog is nice and helps avoid tracking or accidental taps sometimes.
I used K-9 for a long time until I upgraded my phone and K-9 wasn't supported on the newer version of Android so I switched to Faire mail. Has K-9 been updated to support newer versions of Android?
Particularly the link dialog is nice and helps avoid tracking or accidental taps sometimes.