Hi Richard, this is a common problem across iphones, i'm going to put in 4 different methods of correcting this problem, start with 1 go to 4 and test after each one, if you get to 4 and it is still not working, we will need to do some more evasive procedures, but hopefully this fixes it. Let me know if you have already tried these.
1-Tap Settings > Mail, Contacts, Calendars > your email account > Advanced >Delete from server > Set to Never, under INCOMING SETTINGS.
2–Restart your iOS device.
3-Reset your network settings by going to Settings > General > Reset > Reset Network Settings.
4-Remove your email account and then re-add. To do this, tap Settings > Mail, Contacts, Calendars > Your email account > Delete account. Then add your account by going to Settings > Mail, Contacts, Calendars > Add Account.