K9 > Settings > Global Settings > Network > Background sync: When ‘Auto-sync’ is checked
It's not obvious at all it meant ‘Auto-sync’ in Android’s settings.
Personally I spent a lot of time and efforts just to find it out. Not everybody might be such stubborn.
BTW, what exactly the Android's Setting is this? It is even not mentioned in Documentation.
Different Android versions have different Settings(names, locations). Didn't find any suitable for Nougat.
I'd like to introduce more in-app help to address this. That probably means moving away from PreferenceActivity to PreferenceFragment and then adding the icon to the Toolbar.
You can see an example of that in Android here - see the ? button in the top right.

I've raised the linked issue on the website to improve the documentation.
We can perhaps also find a way to link to that setting in your Android settings.
Thanks @philipwhiuk, the in-app help is what I was thinking myself. Also, the link to that setting in Android settings would solve the problem definitely
Back in gingerbread you could understand, it was hidden behind the overflow buttons, now its called "automatically sync data"
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I'd like to introduce more in-app help to address this. That probably means moving away from PreferenceActivity to PreferenceFragment and then adding the icon to the Toolbar.
You can see an example of that in Android here - see the ? button in the top right.
I've raised the linked issue on the website to improve the documentation.