How to configure Custom emails
To send the submitted data to your custom email (via the default php mail function) you should open the settings of the project and then open the "Forms" tab, then choose the first box under Custom Email:
The first box is for custom email
Then input your email address and the subject of the email in the corresponding fields at the end of the page.
Fill up your Custom email information
After that you should open your project inside the builder and click on the "Form Settings" button in the settings of the column which contains the form, then from the right sidebar open the "Settings" tab and choose "Custom Email" (you can keep list id field empty).
To open the form settings click on the "Envelope icon" in the top blue bar of the form
To change field's name simply hover on the field inside the builder and click on the EDIT button on it, the from the popup you can change field's name, placeholder and set if it's required or not.