Mail Background Activity draining battery iOS 11.4

So mail has used 45% of 70% or 32% of your total battery usage. That may or may not be normal depending on how much mail you have gotten, whether it's Wi-Fi or cellular data, and if cellular how strong the signal was. Note that background app refresh has nothing to do with mail if you have it set to Fetch. With Fetch enabled mail will wake up and download all new mail every 15 minutes. That is not considered a background operation.

You can try restarting the phone (Restart your iPhone, iPad, or iPod touch - Apple Support) if you think your mail volume does not account for that amount of drain.

There is no way to completely eliminate background activity; mail Fetch will run in background whether you have background app refresh enabled or not. As will Notifications, incoming text messages, phone calls, app alerts, Reminders, Alarms, Timers, photo sync, Music and podcast apps, etc. Background App Refresh specifically applies to apps that independently, without any external stimulus, "want" to run to do some sort of processing such as updating their database periodically. Very few apps actually do this, but it is an option that developers may choose to use. But any app that can respond to an external stimulus can still run in background to process it regardless of the setting.

Why don’t you try set Fetch to Manual for mail in your Settings?