There are five possible reasons that have triggered this error:
1. You have enabled the autoresponder feature, but you haven’t assigned the email field in the right place. Go to the Settings → Notifications → Form User (Sender) Options section of your form and make sure the email field is assigned correctly in To, under Autoresponder.
2. You have enabled the autoresponder feature, but your email field is not required, allowing your form users to skip it. Return to the Form Editor section of your form and set the email field as required to prevent this from happening again.
3. You have enabled the Send a copy of the completed form to the sender feature, but you haven’t assigned the email field from the dropdown list. Go to Notifications → Form User (Sender) Options and make sure the email field is assigned under Submission Copy.
4. In Settings – Notifications – Form Owner make sure that at From Email you have selected our email server: noreply@captainform.com
5. You have added a short text field on the form instead of our email field.