Where is the logout button in the Android app? Google and the forums return no results.
There’s no login to K9, hence no logout. It’s the same with the Gmail app. With Fairmail, the paid version has the option to enable an idle-time based logout but the free version is like the Gmail and K9 apps. Most email clients (desk/laptop/mobile device) that I’ve encountered depend on system-level (generally idle timer based) locks rather than locking at the application level.
@Abdiel_Rosario Welcome to the forum.
The only way to actually terminate and kill the app is through the system settings (Settings > Apps > Recently opened apps > K-9 > Force stop)
If you rather have an automated termination, @njeyaakili has presented a solution (though I prefer Bluemail and R2Mail2). If you insist on K-9, you may want to have a look at Tasker or Automate. You can alway create a flow that automatically stops and starts other apps (in this case K-9) at specific times or in regular intervals.
With “logout”, do you mean removing the account from K-9?