Hello seasons,
To help resolve issues with your email, check to see if this issue occurs with any other email accounts. Try sending the email in your email client and through the webmail to see if you can duplicate the issue. Make sure the iPhone is up-to-date: Go to Settings > General > Software Update.The screen shows the currently installed version of iOS and whether an update is available.To turn off automatic updates, go to Settings > General > Software Update > Automatic Updates.If this only occurs in the Mail app, delete the app and redownload it:If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple SupportDelete the app, then redownload itDelete the app. When you do this, you might lose data that was stored in the app. Then redownload the app. Then check to see if the problem with the app is fixed.
Best Regards.