Question

I am currently working on creating my own SMTP server for a project I am working on.

It is all pretty much working except for when I enter data, I get a response from something else that isn’t my program.

For doing the testing I am running my program which binds to IPAddress.Any and on port 25 for smtp. I am then using telnet from my local PC to test I am sending the correct responses. While doing this I turned on debug and stepping through the code to see what my program was doing.

• When I connect I send a 220 with my domain name
• When telnet sends the EHLO I send back a 250 with my domain name
• From telnet I send MAIL FROM: [e-mail] and I send back 250 2.1.0 OK
• From telnet I send RCPT TO: [e-mail] and I send back 250 2.1.5 OK

Now this is where the strange things start happening,
From telnet I send DATA ready for the main email message and in the code I am supposed to be sending a 354 End data with <CR><LF>.<CR><LF> but instead I get a response of 354 please start mail input.

From all the steps before data visual studio went into the debug mode so I could step through but upon entering DATA visual studio didn’t respond as if my program never received anything and it instead went somewhere else.

I have no idea where this is coming from, I have no other SMTP server running on my PC.

Below is the code I am using

private void processSmtpReceived(TcpClient client)
        {
            stream = client.GetStream();
            reader = new StreamReader(stream);
            writer = new StreamWriter(stream);
            writer.NewLine = "\r\n";
            writer.AutoFlush = true;

            //writer.WriteLine("220 localhost -- Fake proxy server");
            string line = "";
            string message = "";
            string subject = "";
            bool readingData = false;
            if (client.Connected)
            {
                writer.WriteLine("220 localhost -- Fake proxy server");

                while ((line = reader.ReadLine()) != null && reader != null)
                {
                    line = line.Replace("\b", "");
                    if (readingData)
                    {
                        message += line;
                        if (line.Contains("Subject:"))
                        {
                            subject = line;
                        }
                        else if (line == ".")
                        {
                            writer.WriteLine("250 2.0.0 OK");
                            readingData = false;
                        }
                    }
                    else if (line.Contains("EHLO"))
                    {
                        writer.WriteLine("250 OK localhost-workgroup");
                    }
                    else if (line.Contains("MAIL FROM"))
                    {
                        writer.WriteLine("250 2.1.0 OK");
                    }
                    else if (line.Contains("RCPT TO"))
                    {
                        writer.WriteLine("250 2.1.5 OK");
                    }
                    else if (line.Contains("DATA"))
                    {
                        writer.WriteLine("354 End data with <CR><LF>.<CR><LF>");
                        readingData = true;
                    }
                    else if (line == ".")
                    {
                        writer.WriteLine("250 2.0.0 OK");
                    }
                    else if (line == "QUIT")
                    {
                        writer.WriteLine("221 2.0.0 Bye");
                        Console.WriteLine("Message: " + message);
                    }
                    else
                    {
                        writer.WriteLine("250 OK");
                    }
                    Console.WriteLine("Received: {0}", line);
                }
            }

As you can see from the code the response of start mail input is not in my code so no idea where this is coming from.

Thanks for any help you can provide

UPDATE
Have just found out something that is a bit weird. If I run the SMTP server and telnet on the same machine i.e. telnet localhost 25 it works correctly, and I get the 354 end with <CR><LF>.<CR><LF>, however if I telnet from a separate server i.e. telnet 192.168.1.74 25 then I get the 354 please start mail input. This is the same whether it is on Windows or Linux, I have no idea why it is generating this response as there is no where in the code that response is entered, and there is no smtp server running as when I stop my program and try and connect to telnet on port 25 I get the connection failed message.

UPDATE 2
As a test I changed the code so instead from telnet entering DATA for the message I enter MYTEST, when I used MYTEST my program receives the message that I am sending out and my program works as expected, its only when in the program I use DATA, which is what is expected for SMTP that my program doesn’t receive it, and I get the response from something else. I have done a network scan to see if there is anything else running as an SMTP server somewhere on my network but nothing is found.

UPDATE 3
I’ve also just found out that the problem I am having with sending DATA not being received by my program through telnet is only on Windows to remote Windows. If I telnet from a linux server to the smtp server running on Windows it works fine, its only if I am telneting from windows that I having the problem with the DATA command.

UPDATE 4
Have done a test and found that its not specific to Windows but my desktop and laptop development machines. I have installed a blank VM with Windows 7 and if I telnet to itself, i.e. localhost it works fine, if I telnet from my dev PC to the virtual machine then I get the unexpected response and it doesn’t go via my program. I guess there must be something on both my laptop and desktop that listens to smtp traffic in some way, not sure how as when my program isn’t running I’m not able to connect to port 25. The only thing I can think of is I have Symantec Norton 360 installed but I have disabled the firewall, antivirus and antispam and made no difference.

Answer 1

Just found out the problem. I’ve proved that Symantec Norton 360 seems to take the input when DATA is sent via the smtp client. For some reason even disabling norton including the antispam doesn’t seem to make a difference. There is a section in the anti spam settings for protected ports and it has port 25 added but it doesn’t seem to let me want to remove the port. However, as my program allows the smtp port to be configurable, if I change it to a different port i.e. port 26 then my program works fine.

Thanks for your help and suggestions.