C# toplu mail yollayabilmek için Güzel bir kod paylaşıyorum... Umarım işinize yarar. 

/// <summary>

        /// Send Mails

        /// </summary>

        /// <param name="mailTo">";" seperated mail adrresess</param>

        /// <param name="MailCC">";" seperated mail adrresess</param>

        /// <param name="mailSubject"></param>

        /// <param name="mailBody"></param>

        /// <param name="isHighPriority"></param>

        /// <returns></returns>

        private static string SendMail(string mailTo, string MailCC, string mailSubject, string mailBody, bool isHighPriority)

        {

            try

            {

                MailMessage m = new MailMessage();

                m.Body = mailBody;

                m.Subject = mailSubject;

                MailAddressCollection mToAdrCol = new MailAddressCollection();

  //Gönderilcek çoklu mail adresslerini formata uygun hale getiriyoruz.

                string[] mailToAry = mailTo.Split(";".ToCharArray());

                foreach (string m_to in mailToAry)

                {

                    m.To.Add(m_to);

                }

                string[] mailCCAry = MailCC.Split(";".ToCharArray());

                if (MailCC != "")

                {

                    foreach (string m_to in mailCCAry)

                    {

                        m.CC.Add(m_to);

                    }

                }

                m.From = new MailAddress("info@alkansuzen.com", "Admin - Teşekkür Mesajı");

                m.IsBodyHtml = true;

                m.BodyEncoding = System.Text.Encoding.UTF8;

                if (isHighPriority)

                    m.Priority = MailPriority.High;

                m.SubjectEncoding = System.Text.Encoding.UTF8;

                SmtpClient smtp = new SmtpClient();

                smtp.DeliveryMethod = SmtpDeliveryMethod.Network;

                smtp.Host = ConfigurationManager.AppSettings["SMTP_HOST"];

                smtp.UseDefaultCredentials = false;

                System.Net.NetworkCredential nc = new System.Net.NetworkCredential("username", "password", "domain");

                smtp.Credentials = nc;

                smtp.Send(m);

            }

            catch (Exception exc)

            {

                return exc.Message;

            }

            return "SENT";

        }

Csharp email