I think it is Settings>Battery>Unmonitored Apps (bottom of page)>Add (3 dots upper right)
This will not be enough to fix (I had it setup this way and it was not working : with this the phone will not try to limit battery use for Nine only).
The problem is while the screen is off, Nine will stop receiving push e-mail (imho the system prevents Nine to use Wifi / Cellular network). To stop this from happening, go to :
1) Settings
2) Device Maintenance (I'm translating from French so I'm not sure of the English words)
3) Touch "Battery"
4) Touch "Battery usage" (it's next to the battery use % big circle)
5) Touch the 3 dots menu and select "Optimise battery usage"
6) In the drop down list at the top select "All applications"
7) Find nine in the app and uncheck the check box (or unswitch the selector, it must be off)
And then Nine will behave as before the nougat update.
Note : This is a problem with Nine (other mail client doesntt behave that way, so I guess Nine doesn't request the needed power profile to work correctly without this tweak on our S7 / S7 Edge)