“现有连接被强制关闭”在 .NET 2.0 中发送电子邮件时出错

Question

我有 asp.net 网站。我正在通过 SMTPClient 发送电子邮件。有时我会收到此错误：

 InnerException = System.IO.IOException: Unable to read data from the transport connection: 
    An existing connection was forcibly closed by the remote host. ---> 
       System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   --- End of inner exception stack trace ---
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.DelegatedStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.BufferedReadStream.Read(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine)
   at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller)
   at System.Net.Mail.SmtpReplyReader.ReadLine()
   at System.Net.Mail.CheckCommand.Send(SmtpConnection conn, String& response)
   at System.Net.Mail.DataStopCommand.Send(SmtpConnection conn)
   at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
   at System.Net.ClosableStream.Close()
   at System.Net.Mail.MailWriter.Close()
   at System.Net.Mail.SmtpClient.Send(MailMessage message).

代码：

 SmtpClient smtpClient = new SmtpClient(ConfigurationManager.AppSettings["smtpserverwwwpo"]);
    try
    {
        NetworkCredential basicCredential =
            new NetworkCredential(ConfigurationManager.AppSettings["smtp_user_name"], ConfigurationManager.AppSettings["smtp_password"]);
        smtpClient.UseDefaultCredentials = false;
        smtpClient.Credentials = basicCredential;
    }
    catch
    {
        smtpClient.UseDefaultCredentials = true;
    }

    System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();
    msg.Subject = subject;
    msg.To.Add(new MailAddress(someEmail));
    msg.From = new MailAddress(ConfigurationManager.AppSettings["from_email_address"]);
    msg.Headers.Add("Content-Type", "text/html");
    msg.BodyEncoding = System.Text.Encoding.UTF8;

try{
        smtpClient.Send(msg);
} 
catch(Exception ex)
{
 //write log 
}

}

什么原因会导致它？

Answer 1

MailMessage 是一次性的。您可以尝试将其包装在 using 语句中。

using(var message = new MailMessage {Subject ="...", Body="..."})
{
   message.To.Add("email address");
   new SmtpClient().Send(message)
}

Answer 2

我确实相信它与您发布的实际代码关系不大，而与您所连接的服务器所解释的请求的数量和内容有关。强制关闭连接是一种故意行为，而不是随机错误，这意味着服务器是根据满足某些条件来选择这样做的。

正如上面所建议的，也许您正在达到一些您不知道的极限；或者，远程服务器可能特别繁忙并关闭连接（并且不会返回针对这种情况的正确错误代码）。

Answer 3

也许您应该将 SmtpClient.EnableSsl 设置为 false。

Answer 4

您在哪里分配 SMTP 客户端的主机名...？

如果您更改代码以使用以下内容，不确定是否需要网络凭据，我不熟悉您的规格，但我使用我的东西测试的这段代码只是尝试了一下，它有效。

System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage();
message.To.Add("youremailAddress");
message.Subject = "This is the Subject line";
message.From = new System.Net.Mail.MailAddress("[email protected]");
message.Body = "This is the message body";
System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("yoursmtphost");
smtp.Send(message);

Answer 5

SMTPClient 类实现 IDisposable 接口。
每当 Type 实现 IDisposable 时，您都应该使用 Using(){} 语句。

例如-

using (SmtpClient SmtpServer = new SmtpClient(smtpConfig.SmtpMailServer))
                    {
                       //your code
                    }