Update snippet 'Receive-only SMTP server'

Title

Description

Source code

module SMTP

open System.Net.Sockets
open System.Net
open System.IO

type private Agent<'T> = MailboxProcessor<'T>

type EMail = { Body:string list; Subject:string; From:string; To:string; }

let private emptyEmail = { Body=[];Subject="";From="";To=""; }

type private CheckInbox = 
    | Get of AsyncReplyChannel<EMail list>
    | Add of EMail

let private receiveEmail() = async {
    let endPoint = new IPEndPoint(IPAddress.Any, 25)
    let listener = new TcpListener(endPoint)
    listener.Start()

use! client = listener.AcceptTcpClientAsync() |> Async.AwaitTask
            
    let stream = client.GetStream()
    use sr = new StreamReader(stream)
    use wr = new StreamWriter(stream)
    wr.NewLine <- "\r\n"
    wr.AutoFlush <- true

wr.WriteLine("220 localhost -- Fake proxy server")
   
    let rec readlines email =
        let line = sr.ReadLine()
                
        match line with
        | "DATA" -> 
            wr.WriteLine("354 Start input, end data with <CRLF>.<CRLF>")
            let rec readdata email =
                let line = sr.ReadLine()
                if line = null || line = "." 
                then email
                else 
                    let email =
                        match line with
                        | From l -> {email with From=l}
                        | To l -> {email with To=l}
                        | Subject l -> {email with Subject=l}
                        | Body l when l <> "" -> {email with Body=l::email.Body}
                        | _ -> email
                    readdata email

let newlines = readdata email
            wr.WriteLine("250 OK")
            readlines newlines
        | "QUIT" -> 
            wr.WriteLine("250 OK")
            email
        | _ -> 
            wr.WriteLine("250 OK")
            readlines email 
                
    let newMessage = readlines emptyEmail

client.Close()
    listener.Stop()
    return newMessage }

let private smtpAgent (cachingAgent: Agent<CheckInbox>) = 
    Agent<unit>.Start(fun _ -> 
        let rec loop() = async {
            let! newMessage = receiveEmail()
            cachingAgent.Post (Add newMessage)
            return! loop() }

loop())

let private cachingAgent() =
    Agent.Start(fun inbox -> 
        let rec loop messages = async {
            let! newMessage = inbox.Receive()
            match newMessage with
            | Get channel -> 
                channel.Reply(messages)
                return! loop messages
            | Add message -> 
                return! loop (message::messages) }
        loop [])

type SmtpServer() =
    let cache = cachingAgent()
    let server = smtpAgent cache
    
    member this.GetEmails() = cache.PostAndReply Get