Hello Claire-simm,
To help resolve issues with your email, check to see if this issue occurs with any other email accounts. Try sending the email to your email client and through the webmail to see if you can duplicate the problem.
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.
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 Support
Delete 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.
We’re recommending the steps above because they directly address the issue you initially posted about: emails appear garbled on your iPhone.
If the original problem persists after trying the previous steps, please provide us with specifics on what you see for more help, and we will continue from there.
Cheers!