Email syncing on WiFi only
Caf:
I had what I thought was the same issue upon receiving the 6.0.1 update this past week on my Galaxy  S5. However, I realized that I was plugging my phone in when I returned to the office which was disabling power saver mode which I typically leave on. After turning this off, I was able to sync over the mobile network. Interestingly, changing the doze settings had no effect (I tried them on and off with the power saver). I would see if your user leaves the power saver mode on as that was not explicitly mentioned above.