A recipient must be specified. I encounter this error when I want to use ASP.NET to send email

Question

protected void ButPwd_Click(object sender, EventArgs e)
{
    string mainconn = ConfigurationManager.ConnectionStrings["AssDatabaseConnectionString"].ConnectionString;
    SqlConnection sqlconn = new SqlConnection(mainconn);
    string sqlquery = "select UserEmail,UserPassword from [dbo].[User] where UserEmail=@Email";
    SqlCommand sqlcomm = new SqlCommand(sqlquery, sqlconn);
    sqlcomm.Parameters.AddWithValue("@Email", TxtEmail.Text);
    sqlconn.Open();
    SqlDataReader sdr = sqlcomm.ExecuteReader();
    if(sdr.Read())
    {
        string username1 = sdr["UserEmail"].ToString();
        string password = sdr["UserPassword"].ToString();



        MailMessage mm = new MailMessage();
        mm.From = new MailAddress("dylanng2019@gmail.com");
        mm.Subject = "Your Password is !";
        mm.Body = string.Format("Hello : <h1>{0}</h1> is your Email <br /> Your Password is <h1>{1}<h1>",username1,password);
        mm.IsBodyHtml = true;
        mm.Priority = MailPriority.High;


        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.EnableSsl = true;
        NetworkCredential nc = new NetworkCredential();
        smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
        nc.UserName = "dylanng2019@gmail.com";
        nc.Password = "XohImNooBHFR0OVvjcYpJ3NgPQ1qq73WKhHvch0VQtg=";

        smtp.UseDefaultCredentials = true;
        smtp.Credentials = nc;
        smtp.Port = 587;
        Labmsg.Text = "Your Password has been sent to " + TxtEmail.Text;
        Labmsg.ForeColor = Color.Red;
        smtp.Send(mm);
    }
    else
    {
        Labmsg.Text = TxtEmail.Text + ". This Email is not exist in the database";
        Labmsg.ForeColor = Color.Red;
    }
}

Answer 1

mm.To.Add("recipient@address.com");     //Add this line to your code