iOS 11.3.1 Mail app doesn't shown new mail until the iPhone is rebooted - any cure?
A couple of things to try


Delete the Account from your phone and re add it.
Go into Settings > General > Reset and reset the network settings


You will lose the WiFi networks you have already been on and authenticated to but all you have to do is re-connect and re-authenticate.


Failing the above I would back up the phone (password protect the backup so you don't lose your saved passwords.) Wipe it (Settings > General > Reset > Erase all content and settings) and then restore from the backup you just made.