C＃發送郵件時出錯：連接請求是在已經連接的套接字上進行的

Question

我見過很多從c＃代碼發送郵件的例子，我遵循最流行的使用System.Net.Mail.SmtpClient的方法。我能夠首次發送郵件，但之後會發出錯誤說明連接請求發生在已連接的套接字上。

我使用下面的代碼來發送郵件：

using (MailMessage mail = new MailMessage()) 
{ 
    using (SmtpClient mailer = new SmtpClient("smtp.server.com")) 
    { 
     Console.Write("Sending mails..."); 
     mail.From = new MailAddress("from@mail.com"); 
     mail.To.Add("myaddress@mail.com"); 
     mail.Subject = "test-subject"; 
     mail.Body = "test-body"; 
     mailer.Port = 25; 
     mailer.Credentials = new System.Net.NetworkCredential("from@mail.com", "from-password"); 
     mailer.EnableSsl = true; 
     try 
     { 
      mailer.Send(mail); 
      Console.WriteLine("Mail sent"); 
     } 
     catch (Exception e) 
     { 
      Console.ForegroundColor = ConsoleColor.Red; 
      Console.WriteLine("\rCannot send mail, an error occured : {0}", e); 
      Console.ResetColor(); 
     } 
    } 
} 

錯誤消息：

Cannot send mail, an error occured : System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: A connect request was made on an already connected socket 200.200.200.200:25 
    at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress) 
    at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Int32 timeout, Exception& exception) 
    --- End of inner exception stack trace --- 
    at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6, Int32 timeout) 
    at System.Net.PooledStream.Activate(Object owningObject, Boolean async, Int32 timeout, GeneralAsyncDelegate asyncCallback) 
    at System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate asyncCallback) 
    at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout) 
    at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint) 
    at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint) 
    at System.Net.Mail.SmtpClient.GetConnection() 
    at System.Net.Mail.SmtpClient.Send(MailMessage message) 
    --- End of inner exception stack trace --- 
    at System.Net.Mail.SmtpClient.Send(MailMessage message) 
    at TestApp.Program.Main(String[] args) in D:\Desktop\TestApp\TestApp\Program.cs:line 290 

兩個MailMessage和內部using條款正如你所看到的，我一直實例SmptClient這樣如果發生任何錯誤，連接將自動關閉。但它在正確引導後第一次發送郵件，之後所有其他嘗試都失敗。

SMTP服務器位於代理之後，防火牆客戶端正在我的系統上運行。代理不是一個問題，在我看來這裏。

代碼中是否有錯誤？我怎樣才能使它工作？

我已經嘗試使用Outlook API，但在發送每封郵件之前需要用戶進行身份驗證。我不想要這種行爲。 PS：我刪除了一些細節，如IP地址和服務器名稱，在這種情況下無關緊要。

在此先感謝:)

Answer 1

也許您應該升級到NET 4.0。 This question Microsoft Connect上解釋說，SMTPClient沒有正確關閉連接，這個問題已被固定在4.0

有人使用屬性MaxIdleTime設置爲1取得了一些成功，但在任何情況下，它似乎只有當連接數超過郵件服務器允許的數量時纔會向前移動問題。