Custom email domains are only available on the Professional and Business plans.

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How to enable custom email domain

Go to the form you want to enable the custom email domain for and navigate to the Settings tab.
Navigate to Email configuration section, and enable the toggle. The Respondent email notifications toggle will need to be enabled as well.
From there, you can update the SMTP settings by clicking on the Edit configuration button.
Done, now every time a customer submits a message to your form, they will get an auto response email notification from your custom email domain.