send mail to yahoomail

i am sending group mail to my all user email id which is stored in my database , i got some problem first it will take more time to send mails and it will send only mail to gmail  but i have yahoomail id also but it is not delivered to other mail here is my code:




using System.Data.SqlClient;
using System.Web.Mail;
using System.Collections.ObjectModel;


SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ncon"].ConnectionString);
protected void Button2_Click(object sender, EventArgs e)
    {


        con.Open();
        SqlCommand cmd = new SqlCommand("select nemail from ncreate", con);
        SqlDataReader red = cmd.ExecuteReader();

        string MyVar = "";
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