This is a well-known and well-documented issue. In Android 7.x you need to
"un-optimize" K9 in the battery settings.
Settings -> Battery -> three dot menu, Battery optimization Select All
apps and make K9 Not optimized. Problem solved.
On Wednesday, May 17, 2017 at 11:29:10 AM UTC-5, Terry S
K9 has worked well for many years but after my last 2 Android updates it seems
to quit checking my email unless I open the app. After I open K9 and check my
mail it may work fir an hoir then it seems to go a sleep. Any one else
experiencing this. I have a Samsung Galaxy S7 on Andriid version 7.0