The easy and reliable solution to mail problems on any version of iOS has always been to simply:
- remove the account from the device
- power off the device
- power on the device
- add the mail account back
This will solve mail problems simply and easily nearly ever time when IMAP or other modern server/cloud protocol is used for the account. It's honestly not worth troubleshooting when this will solve the issue immediately.
* The only reason not to use this solution is if you are still in 2016 using POP email! If you are, then you need to make sure that your POP mail has been saved to another device because removing the account will also remove the emails.