Fetch new mail only while on Wifi (but not while on cellular data)
Go to the settings app:


Cellular


Turn of mail for using cellular data. (Swipe up to see the M section).  Unfortunately, this will disable all syncing over cellular data, and it will prevent you from even a manual fetch of email while on cellular data.  So, you'll need to twiddle with things whenever you want to do a manual fetch while on cellular data.