Great email client I have used K9 for years  since my Galaxy S1 and now on my Google Pixel with Android 7.1. I did notice sync randomly being disabled on the new device and found this is due to doze (battery save) in the kernel. Once I removed the app from doze it now works great as usual and with three accounts that sync at 5 10 and 30 minute intervals only uses about 6℅ battery life in a 14 hour period. I actively use K9 throughout the day to send mail as well  usually large mail with pictures. Best by far of all clients.
December 01 2016