Hello butterfly55,
Thank you for using Apple Support Communities. We want to help troubleshoot this unexpected issue with your iPhone. Allow us to assist by recommending some steps. Follow the steps below:
1.) Close out the Mail app: Close an app on your iPhone or iPod touch - Apple Support
2.) Refresh things by restarting your iPhone: Restart your iPhone - Apple Support
3.) If these steps don't help and this issue is only affecting one mail account, try removing the affect mail box and re-add it back to your iPhone. This can sometimes help resolve unexpected mail issues: Add and remove email accounts on iPhone - Apple Support
Also, if you have any pending updates available, it would be a good idea to install them.
This can usually help with unique issues, since updates include fixes. Just in case, this link can guide you through the update process: Update your iPhone or iPad - Apple Support
1.)Back up your device using iCloud or your computer.
2.)Plug your device into power and connect to the internet with Wi-Fi.
3.)Go to Settings > General, then tap Software Update.
4.) If you see more than one software update option available, choose the one that you want to install.
5.) Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do: If you forgot your iPhone passcode - Apple Support
Thank you for using Apple Support Communities. Take care.