The parameter 'addresses' cannot be an empty string

SeqNo DocID ApproveID ApproveBy
82  20  3   john
83  21  1   john
84  18  2   kety
85  19  1   emel

DocID DocName Uplaodedfile UserEmail       DocType DepType ApproveID
1      ABC      def.pdf    abcdef@gmail.com  pdf      hr      (In this i set dropdown values are (approve/reject/pending)

protected void Button1_Click(object sender, EventArgs e)
    {

            string connStr = 
            ConfigurationManager.ConnectionStrings["mydms"].ConnectionString;
          SqlConnection mySQLconnection = new SqlConnection(connStr);
          if (mySQLconnection.State == ConnectionState.Closed)
          {
              mySQLconnection.Open();

              for (int i = 0; i < Repeater2.Items.Count; i++)
              {
                  DropDownList DropDownListcontrol = 
                ((DropDownList)Repeater2.Items[i].FindControl("DropDownList4"));
                  Label DocId = ((Label)Repeater2.Items[i].FindControl("DocId"));



                  SqlCommand cmd = new SqlCommand("approveddd", mySQLconnection);
                  cmd.CommandType = CommandType.StoredProcedure;


                  cmd.Parameters.Add("@DocID", SqlDbType.Int).Value = 
                  Convert.ToInt32((DocId.Text));

                  cmd.Parameters.Add("@ApproveID", SqlDbType.Int).Value = 
              Convert.ToInt32(DropDownListcontrol.SelectedValue);
                  cmd.Parameters.Add("@ApproveBy", SqlDbType.VarChar, 50).Value = 
                   (Session["Login2"]);
                  try
                  {
                      cmd.ExecuteNonQuery();
                      string emailId = 
              ((Label)Repeater2.Items[i].FindControl("Label2")).Text;
                      SendEmailUsingGmail(emailId);
                  }
                  catch (Exception ex)
                  {
                      Supvisor.Text=(ex.Message);
                  }
                  cmd.ExecuteNonQuery();
                  //UPDATE APPPROVEID IN DOCUMENTINFO TABLE
                  //DMSLIB.Doc myDoc = new DMSLIB.Doc();
                  //myDoc.MarkDocAs(Convert.ToInt16(DocId.Text), 
                 Convert.ToInt32(DropDownListcontrol.SelectedValue));

              }

          }
          else
          {
              Supvisor.Text = "Error";
          }
          if (mySQLconnection.State == ConnectionState.Open)
          {
              mySQLconnection.Close();
          }
                }
    private void SendEmailUsingGmail(string toEmailAddress)
    {
        try
        {
            SmtpClient smtp = new SmtpClient();
            smtp.Credentials = new NetworkCredential("keysketyyyy@gmail.com", 
             "sdsdasd");
            smtp.Port = 587;
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            MailMessage message = new MailMessage();
            message.From = new MailAddress("keysketyyy@gmail.com");
            message.To.Add(toEmailAddress);
            message.Subject = "Write your email subject here";
            message.Body = "write the content of the email here";
            smtp.Send(message);
        }
        catch (Exception ex)
        {
            Response.Write("Error occured: " + ex.Message.ToString());
        }
    }

catch (Exception ex)
        {
            Response.Write("Error occured: " + ex.Message.ToString());
        }

error

The parameter 'addresses' cannot be an empty string.
     Parameter name: addresses