Imports System.IO
Imports System.Threading
Imports System.Net
Imports System.Net.Sockets
Imports System.Text

Public Class Form1
    'Add a TEXTBOX1 to form
    'Add a TEXTBOX1 to form
    'Add a TEXTBOX1 to form
    'Add a TEXTBOX1 to form

    Private Server As Sockets.TcpListener
    Private Port As Integer = 25
    Private Thread As Thread

    Delegate Sub LogCallback(S As String)

    Sub Log(S As String)

        If Me.TextBox1.InvokeRequired Then
            Dim d As LogCallback = AddressOf Log
            Me.Invoke(d, S)
        Else
            Me.TextBox1.AppendText(S.Replace("n", vbCrLf))
        End If
    End Sub

    Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles MyBase.Shown
        Me.Text = String.Format("Listening port {0} v{1}", Port, AppVersion)

        Thread = New Thread(New ThreadStart(AddressOf Listening))
        Thread.Start()
    End Sub

    Async Function Listening() As Task
        Server = New Sockets.TcpListener(Net.IPAddress.Any, Port)
        Server.Start()

        Log("Starting listening on port 25nn")

        Do
            Dim HandleThread As New Thread(New ParameterizedThreadStart(AddressOf HandleConnection))
            HandleThread.Start(Await Server.AcceptTcpClientAsync)
        Loop

    End Function

    Async Function HandleConnection(TcpClient As TcpClient) As Task

        Dim Stream As NetworkStream = TcpClient.GetStream

        Log(String.Format("nClient connected: {0}n", TcpClient.Client.RemoteEndPoint.ToString))
        Await R(Stream, String.Concat("220 your.mail.server.com Service ready", vbCrLf))

        While True

            Do
                Dim BuffSize As Integer = 4096
                Dim Buffer As Byte() = New Byte(BuffSize - 1) {}
                Dim BytesRead As Integer

                BytesRead = Await Stream.ReadAsync(Buffer, 0, Buffer.Length)

                If BytesRead = 0 Then Exit Function


                Dim Msg As String = Encoding.ASCII.GetString(Buffer.Take(BytesRead).ToArray)

                Log(String.Concat("<<< ", Msg))

                If Msg.StartsWith("EHLO") Then
                    Await R(Stream, "502 Command not implemented")
                ElseIf Msg.StartsWith("HELO") Then
                    Await R(Stream, "250 your.mail.server.com")
                ElseIf Msg.StartsWith("MAIL") Or Msg.StartsWith("RCPT") Or Msg.StartsWith("Date") Or Msg.StartsWith("From") Then
                    Await R(Stream, "250 OK")
                ElseIf Msg.StartsWith("DATA") Then
                    Await R(Stream, "354 Start mail input; end with <CRLF>.<CRLF>")
                ElseIf Msg.StartsWith("From") Or Msg.StartsWith("Received") Then
                    Await R(Stream, "250 OK")
                ElseIf Msg = vbCrLf Then
                    Await R(Stream, "250 OK")
                ElseIf Msg.StartsWith("QUIT") Then
                    Await R(Stream, "221 BBN-UNIX.ARPA Service closing transmission channel")
                    Stream.Close()
                    TcpClient.Close()
                    Exit Function
                Else
                    Await R(Stream, "502 Command not implemented")
                End If
            Loop While Stream.DataAvailable

        End While

    End Function

    Private AppVersion As Integer = 8

    Async Function R(Stream As NetworkStream, Message As String) As Task

        Dim ResponseBuffer As Byte() = Encoding.ASCII.GetBytes(String.Concat(Message, vbCrLf))
        Await Stream.WriteAsync(ResponseBuffer, 0, ResponseBuffer.Length)

        Log(String.Concat(">>> ", Message, vbCrLf, vbCrLf))
    End Function

End Class