.net smtp listener

So I put this together out of sheer curiosity, although I do agree with tgerbert that you may find an ready-built solution more easy to apply. My hope is that if you wanted to do the code yourself, then this will give you a start. This example is *extremely* basic. There is only one iteration on the "server;" in reality, I would expect this to be in a loop waiting for new connections. There is no serious error checking, and I may not be following the SMTP protocol strictly (I did intend to). There is no verification of the recipient as I'm not sure how you'd want to verify that.

Server

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;

namespace ConsoleApplication42
{
    class Program
    {
        static void Main(string[] args)
        {
            TcpListener listener = new TcpListener(IPAddress.Loopback, 8000);
            TcpClient client;
            NetworkStream ns;
            
            listener.Start();

            Console.WriteLine("Awaiting connection...");
            client = listener.AcceptTcpClient();
            Console.WriteLine("Connection accepted!");

            ns = client.GetStream();

            using (StreamWriter writer = new StreamWriter(ns))
            {
                writer.WriteLine("220 localhost SMTP server ready.");
                writer.Flush();

                using (StreamReader reader = new StreamReader(ns))
                {
                    string response = reader.ReadLine();

                    if (!response.StartsWith("HELO") && !response.StartsWith("EHLO"))
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    string remote = response.Replace("HELO", string.Empty).Replace("EHLO", string.Empty).Trim();

                    writer.WriteLine("250 localhost Hello " + remote);
                    writer.Flush();

                    response = reader.ReadLine();

                    if (!response.StartsWith("MAIL FROM:"))
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    remote = response.Replace("RCPT TO:", string.Empty).Trim();
                    writer.WriteLine("250 " + remote + " I like that guy too!");
                    writer.Flush();

                    response = reader.ReadLine();

                    if (!response.StartsWith("RCPT TO:"))
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    remote = response.Replace("MAIL FROM:", string.Empty).Trim();
                    writer.WriteLine("250 " + remote + " I like that guy!");
                    writer.Flush();

                    response = reader.ReadLine();

                    if (response.Trim() != "DATA")
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    writer.WriteLine("354 Enter message. When finished, enter \".\" on a line by itself");
                    writer.Flush();

                    int counter = 0;
                    StringBuilder message = new StringBuilder();

                    while ((response = reader.ReadLine().Trim()) != ".")
                    {
                        message.AppendLine(response);
                        counter++;

                        if (counter == 1000000)
                        {
                            ns.Close();
                            return;  // Seriously? 1 million lines in a message?
                        }
                    }

                    writer.WriteLine("250 OK");
                    writer.Flush();
                    ns.Close();
                    // Insert "message" into DB
                    Console.WriteLine("Received message:");
                    Console.WriteLine(message.ToString());
                }
            }

            Console.ReadKey();
        }
    }
}

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Client

using System.Net;
using System.Net.Mail;

namespace ConsoleApplication43
{
    class Program
    {
        static void Main(string[] args)
        {
            MailMessage message = new MailMessage("ma@example.com", "pa@example.com");
            SmtpClient client = new SmtpClient(IPAddress.Loopback.ToString(), 8000);

            message.Body = "Don't forget to pick up some milk from the market.";

            client.Send(message);
        }
    }
}

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Here are the references I used in creating the example above:

    https://helpdesk.islandnet.com/pep/smtp.php
    http://www.networksorcery.com/enp/protocol/smtp.htm




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