One way to extend your iPhone's battery life is to limit how often your phone checks for new email. Here's how to prevent iPhone Mail from automatically checking for new mail, and how to schedule accounts (or specific folders) to check for new messages with iOS 12 and later.
How to Enable Email Fetching or Pushing on iPhone
You can change the Mail Fetch New Data settings to set how often iPhone Mail checks your accounts for new messages.
Open the Settings app.
Go to Email > Accounts.