I wasn’t suggesting deleting the app. I was talking about completely quitting the app. Most of the time there is no need to force close an app unless it freezes, or misbehaves.
Force closing an app on the iPhone X or newer, involves, swiping up from the bottom of the screen and pausing when you see the previews of the open apps on the device. Then swipe to the right to get to the app you want to close. Swipe that app preview up and off the top off screen to close it.
On earlier iPhones, you double tap the home button to get to the app previews.
This is how Apple describes the process.
Close an app on your iPhone or iPod touch - Apple Support
Force closing the mail app would have caused the app to refresh when you launched it again. Your mail issues seems to have fixed itself, so you are good to go now.
If you want to learn how to delete an app, read this Apple support article.
Delete apps on your iPhone, iPad, and iPod touch - Apple Support
This one explains how to download apps again.
Redownload apps and games from Apple - Apple Support
There is absolutely no reason for you to delete the mail app. I just posted this information because you inquired. If you have something you need help with, don’t hesitate to post in the community again.