I do the same as John Yester (though I use the Google InBox app.) However, there are times in the past when the Mail app has spiked in the battery list. In the past on iOS 10.x, force quitting the Mail app (which I do not often use) has usually solved that problem, though, of course, it can take a while for the battery stats to show a difference.
The Google InBox app, or even the stock Google Gmail app, is so much better for me with Gmail accounts, mostly because I am only notified when a message comes in to my Primary label (as opposed to Social, Promotions, Updates and Forums.) Because the stock Mail app doesn't distinguish between those types of messages, I would get way to many notifications during the day. Using VIP for notifications wouldn't help, because not all of the messages that I receive in Primary are in my contacts list.