发送邮件时出现 C# 错误：在已连接的套接字上发出了连接请求

Question

我见过很多从 c# 代码发送电子邮件的例子，我正在遵循最流行的使用 System.Net.Mail.SmtpClient 的方法。我能够第一次发送邮件，但之后它给出了一个错误提示 在已连接的套接字上发出了连接请求。

我正在使用以下代码发送邮件：

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();
        }
    }
}

错误信息是：

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

如您所见，我在using 子句中保留了MailMessage 和SmptClient 的实例，这样如果发生任何错误，连接将自动关闭。但它在启动后第一次正确发送邮件，之后的所有其他尝试都失败了。

SMTP 服务器位于代理后面，防火墙客户端正在我的系统上运行。代理不是问题，我认为就在这里。

代码有错误吗？我怎样才能让它工作？

我曾尝试使用 Outlook API，但这需要用户在发送每封邮件之前进行身份验证。我不想要这种行为。

PS : 我已经删除了一些在这种情况下无关的细节，比如 IP 地址和服务器名称。

提前致谢:)

Answer 1

您可能应该升级到 NET 4.0。 Microsoft Connect 上的This question 解释说 SMTPClient 没有正确关闭连接，此问题已在 4.0 中修复

有人使用属性 MaxIdleTime 设置为 1 取得了一些成功，但无论如何，当连接数超过邮件服务器允许的数量时，问题似乎只是向前发展.