I see that you're having an issue with emails returning to your inbox after deleting them. I can understand your concern and I'd be happy to help.
Let's start by restarting your iPhone and then test to see if the issue remains:
Restart your iPhone, iPad, or iPod touch
If the issue still remains, let's make sure you're using the latest version (11.1.2) of software on your iPhone. If not go ahead and update:
Update the iOS on your iPhone, iPad, or iPod touch
After checking on this and testing, if the issue still remains, reply back with the answers to the following questions:
1. When did this issue begin?
2. Does this happen when you're connected to Wi-Fi or cellular data?
3. Which email provider do you use?
Thank you for using Apple Support Communities.
Have a great day!