sending mail and from=textbox value

I have feedback form .
Name : (txtname)
Email : (txtemail)
feedback :(txtfeedback)
button(btnsend)

i sending mail using this method.

public void sendMailall1(string subject, string body)
    {

        MailMessage mail = new MailMessage();
        mail.To.Add("my other ID");
        mail.From = new MailAddress("mygmailid");
        mail.Subject = subject;
        mail.Body = body;
        mail.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.Credentials = new System.Net.Networ ...