using(SqlConnection conn = new SqlConnection(stringaDiConnessione))
{
   using(SqlCommand cmd = new SqlCommand)
   {
      cmd.CommandText = "SELECT * FROM ...";
      cmd.Connection = conn;
      conn.Open();
      SqlDataReader reader = cmd.ExecuteReader();
      while(reader.Read())
      {
         MailMessage mail = new MailMessage("da me", reader["CampoDelDB"].ToString(), "soggetto", "corpo");
         SmtpClient client = new SmtpClient("server smtp");
         client.Send(mail);
      }
      reader.Close();
   }
}

public void sendEmail_Click(object sender, EventArgs e) 
{ 
        using(OleDbConnection conn = new OleDbConnection("Stringa di connession"))
        {
        using(OleDbCommand cmd = new OleDbCommand())
        {

        cmd.Connection = conn;
        cmd.CommandText = "SELECT * FROM ....";
        OleDbDataReader reader = cmd.ExecuteReader();
        while(reader.Read())
        {
        MailMessage mail = new MailMessage(); 

        //mittente: 
        mail.From = new MailAddress("staff@PalladioViaggi.com", "Staff PalladioViaggi.com"); 

        //Destinatario: 
        string destinatario = String.Format("{0}", reader["CampoDelDB"]);
        mail.To.Add(new MailAddress(destinatario));
         
        //Oggetto: 
        mail.Subject = txtSubject.Value; 
        mail.SubjectEncoding = System.Text.Encoding.GetEncoding("iso-8859-15"); 

        //corpo della mail: 
        mail.Body = ..... ; 
        mail.BodyEncoding = System.Text.Encoding.GetEncoding("iso-8859-15"); 
        mail.IsBodyHtml = true; 

        // invio: 
        SmtpClient server = new SmtpClient(); 
        server.Host = "smtp.palladioviaggi.com"; 
        server.Send(mail); 
        }

        }
        }


        lblAvviso.InnerHtml = "E-mail inviata con successo!"; 


        // salvo tutto nel db 
        DbNewsletter.Insert(); 

        txtNome.Text = ""; 
        txtCognome.Text = ""; 
        //txtTel.Text = ""; 
        txtTo.Text = ""; 
        txtBody.Text = ""; 
        txtSubject.Value = ""; 

}

string connectionString = String.Format("Provider=Microsoft.Jet.OLEDB.4.0;Data Source={0}\newsletter.mdb", Server.MapPath("~/app_data"));

    public void sendEmail_Click(object sender, EventArgs e)
    {
        using (OleDbConnection conn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=/app_data/" + Server.MapPath("newsletter.mdb")))

        {
            using (OleDbCommand cmd = new OleDbCommand())
            {
                cmd.Connection = conn;
                cmd.CommandText = "SELECT * FROM contatti";
                OleDbDataReader reader = cmd.ExecuteReader();
                while (reader.Read())
                {

...
...