Please make the delete key permanent and not disappearing.
The Delete button should be visible after you select one or more emails.
If that is not the case, I have a couple of troubleshooting steps for you that should resolve this issue:
1. If you’re signed in, tap the Settings icon, tap Manage Accounts, then tap Edit, tap Remove
2. Restart your mobile device
3. Uninstall and reinstall the app with the latest version
4. Lastly, switch the internet connection from WiFi to mobile data