A variety of issues could be causing this problem, some common problems are listed here:
Acknowledgement email is not enabled – open your DataPage in edit mode and go to “Destination and Emails” screen of the wizard and make sure that the Acknowledgement email option is enabled.
Correct email field is not selected – ensure that on the same screen mentioned above, the correct field from your data source that is meant to hold the recipient’s email address is selected.
Email “From” field should be valid – an increasing number of email servers block emails that do not have a valid FROM email addresses. You must ensure your FROM email address is valid and can receive email.