send automatic thank you email after feedback form submit

After your feedback submitted to DB then try to sent the mail. I just outlined the code below:

        protected void btnSubmit_Click(object sender, EventArgs e)
        {
            // Submit your Feedback to DB
              // Include your code here
            //After submission sent the mail
        }

In order to sent mail, you can use any mail clients or API(Like mailchimp). If you have any further assistance let me know. :)