sending reply to: address

I have created a form which e-mails info to a mail box.  Working fine except that my host requires the sending e-mail address in the "msgl from" block.
   msg1.From = "formmail@coldxxxxxxdrf.com";
   msg1.To = tbTo.Value;
   msg1.Subject = tbSubject.Value;
   msg1.Body = sb.ToString();
 msg1.BodyFormat = MailFormat.Html;
   msg1.Priority = MailPriority.Normal;
   SmtpMail.SmtpServer = "mail.coldxxxxxxdrf.com";

I had it such that the form picked up the senders e-mail from the form:
   msg1.From = txtEmail.Text;
   msg1.To = tbTo.Value;
   msg1.Subject = tbSubject.Value;
   msg1.Body = sb.ToString();
 msg1.BodyFormat = MailFormat.Html;
   msg1.Priority = MailPriority.Normal;
   SmtpMail. ...