Custom validation email and error message in registration
To validate and check if the user's email address upon registration already exists or is already in use, go to wp-admin> Ultimate Member> Forms> Edit Registration Form> Edit Email-Address field.
Set the Validate to Custom Validation in the field settings and input user_email_details in the Custom Action. Then click on the update button to save changes.
Then use the custom code below:
The code above will also check if the user has inputted the correct email address or made typographical errors upon registration. This way, users will know if they provided an invalid email address or check if they made any errors.