When deleting emails, instead of being completely deleted they actually go into a temporary location called "the trash". The app will clean out the trash bin every 30 days so you have up to a month to recover emails you threw in there.
Sadly, you cannot change settings so that emails deleted from the app is not deleted from the server. The app is directly connected to the server. It does not simply load emails for viewing, it actually shows you your email and allows you to interact with them as if you were viewing them on a web browser. The app is exactly the same as if you were to view the email from a computer.
What you can do is open your phone's email app and open the navigation menu. Select "Trash" (you may need to scroll down the menu a bit but the option should be near the bottom). Any emails you "deleted" within the last 29 days will be in there! Simply select whichever email you want to recover, touch the "three dot" options menu, and select "Move To". You will then be prompted to choose where you want to send your recovered emails. Select "Inbox" if you want to send them back there.
If your email's trash bin is empty, there is nothing you can do.
I hope this helped.