各位好，

我的客戶這邊在發mail時，會出現10分鐘timeout的情況。我寫了一支小程式測試，每半分鐘sendmail一次，都沒有錯誤，但到第20次，也就是10分鐘，就會出現4.4.1 connection timeout的訊息。請看圖片。

底下是我的程式碼。

using System;
using System.Net;
using System.Net.Mail;
using System.Threading;


namespace SendUscMail
{
    class Program
    {
        static void Main(string[] args)
        {

            int t = Convert.ToInt32(args[1]);
            for (int i = 0; i <= 35; i++)
            {
                CreateTestMessage4(args[0], args[2]);
                Console.Write(i.ToString());
                Thread.Sleep(t);
                
            }
            Console.ReadLine();
        }


        public static void CreateTestMessage4(string server,string mailTo)
        {
            MailAddress from = new MailAddress("abc@xxx.com");
            MailAddress to = new MailAddress(mailTo);
            MailMessage message = new MailMessage(from, to);
            message.Subject = "測試發信";
            message.Subject += "-測試發信測試發信測試發信測試發信";
            message.SubjectEncoding = System.Text.Encoding.Default;
            message.Body = @"測試發信";
            message.IsBodyHtml = true;
            message.BodyEncoding = System.Text.Encoding.Default;
            
            try
            {
                {
                    SmtpClient client = new SmtpClient(server);
                    client.UseDefaultCredentials = false;
                    Console.WriteLine("Sending an e-mail message to {0} by using SMTP host {1} port {2}.",
                         to.ToString(), client.Host, client.Port);
                    client.Send(message);
                    message.Dispose();
                }
                
            }
            catch (Exception ex)
            {
                Console.WriteLine("Exception caught in CreateTestMessage4(): {0}",
                      ex.ToString());
            }
        }
    }
}

請問這會是什麼地方的問題? (客戶的 mail server是 exchange 2010)