Cant emails through exchange: An existing connection was forcibly closed by the remote host

问题

I just noticed that some of the email operations in my code have been failing and am trying to fix it up, however I keep getting the following error:

System.Net.Mail.SmtpException: Failure sending mail. ---> 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.SmtpConnection.GetConnection(String host, Int32 port)
   at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port)
   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)

My code is fairly straightforward:

MailMessage mail = new MailMessage();
mail.To.Add(email_to);
mail.From = new MailAddress(email_from);
mail.Subject = email_subject;
mail.Body = email_body;


var client = new SmtpClient(smtp_server_url);
client.Credentials = new NetworkCredential(username, password, domain);
client.Timeout = 30000;
client.Send(mail);

Does anyone have any ideas what could be going on? Our exchange server administrator says she can't see anything in the log.

Also, not that this is a surprise, but the same thing happens when I try to send a message from powershell.

回答1:

Got it!

Essentially it seems that this error (ErrorCode 10054) happens when you connect to a server successfully but then ask it to do something that it is not equipped to do.

In my case here is what happened:

I used to send mail through our front-facing mail server mail.mycompany.com
That is not the address of the exchange server though, it is the address you point your browser to to access outlook online.
This worked fine previously since non-web requests for that address were routed to the exchange server. At some point this policy was changed - in our case because spam management was handed off to AT&T. Of course I was not informed of any of this.
Since there was indeed a server running at that site it would let me connect to it but then drop the connection immediately (or possibly even not allow it to open all the way)
Setting the correct host-name fixed the problem.

回答2:

Usually when I see that it's a configuration on the Exchange server. In our environment, our Network Admins must explicitly allow machines to use the Exchange server as a relay. If the machine my code is running on is not explicitly added to the server, I get this message. Hopefully that helps.

来源：https://stackoverflow.com/questions/1161253/cant-emails-through-exchange-an-existing-connection-was-forcibly-closed-by-the

标签

.net

smtp

exchange-server

system.net.mail