C#发邮件本地测试可以，为什么在服务器端不行？

        

using System.Net.Mail;

        /// <summary>

        /// 发送邮件

        /// </summary>

        /// <param name="emailSub">邮件主题</param>

        /// <param name="emailBody">邮件内容</param>

        /// <param name="toMail">收件者</param>

        /// <returns>返回发送的结果</returns>

        public static string SendMail(string emailSub, string emailBody, string toMail)

        {

            string msg = string.Empty;



            MailMessage mail = new MailMessage();



            mail.From = new MailAddress("a@qq.com");

 

            mail.To.Add(new MailAddress(toMail));



            mail.Sender = new MailAddress("a@c.com");



            mail.Subject = emailSub;



            mail.Body = emailBody;

 

            mail.IsBodyHtml = true;

  

            mail.Priority = MailPriority.Normal;





            SmtpClient sc = new SmtpClient("pop3.c.com");



            sc.Credentials = new NetworkCredential("a@c.com", "xxx");

 

            sc.DeliveryMethod = SmtpDeliveryMethod.Network;

            try

            {

                sc.Send(mail);

                msg = "发送成功！";

            }

            catch (SmtpException e)

            {

                msg = e.StatusCode + "_" + e.Message;

            }

            return msg;

        }

在localhost本机调试时，可以发送邮件，部署到2003服务器上，通过域名访问操作时，获取的情况是发送邮件失败，StatusCode是GeneralFailure

msdn上GeneralFailure的解释是
事务未能发生。当未能找到指定的 SMTP 主机时，会收到此错误。

应该如何设置才行呢？