Even though it’s an app built into iOS, Apple’s native Mail app can easily become a battery hog. Luckily, you can reduce its energy consumption by altering a few app settings.
Go to Settings > Mail. Tap on Accounts and then Fetch New Data. This is usually the culprit.
If an account is set to Fetch, it’ll use more battery life depending on how frequent it is. Set it to a longer time interval, or Manually, to save battery.