Address parameter cannot be an empty string

Question

I am trying to send an asp.net message via email using the System.Net.Mail.SmtpClient class .

However, I get the following exception message:

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

This is my email address to send:

    private void SendEmailUsingGmail(string toEmailAddress)
    {
        try
        {
            SmtpClient smtp = new SmtpClient();
            smtp.Credentials = new NetworkCredential("[e-mail]", 
             "sdsdasd");
            smtp.Port = 587;
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            MailMessage message = new MailMessage();
            message.From = new MailAddress("[e-mail]");
            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());
        }
    }

The exception falls into the catch block SendEmailUsingGmail.

This is the call code:

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();
    }
 }

When the administrator approves / rejects the document, then the data is stored in a database like this

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

now I send emails also when the administrator clicks the button, and then emails the corresponding email identifier as shown in the relay table

 DocID  DocName Uplaodedfile    UserEmail           DocType DepType ApproveID
 1      ABC     def.pdf         [e-mail]    pdf     hr      (In this i set dropdown values are (approve/reject/pending)

Answer 1

message.To.Add(New MailAddress(toEmailAddress));

"toEmailAddress".

Answer 2

, . . , toEmailAddress - .