OAuth を使って Exchange Online に POP で接続する

using Microsoft.Identity.Client;
using System;
using System.IO;
using System.Linq;
using System.Net.Security;
using System.Net.Sockets;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading.Tasks;

namespace PopClientDemo
{
    class Program
    {
        private const string ClientId = "<CLIENT ID OF YOUR APP>";
        private const string Authority = "https://login.microsoftonline.com/organizations";
        private static readonly string[] Scopes = new string[] { "https://outlook.office.com/POP.AccessAsUser.All" };

        static void Main(string[] args)
        {
            var tokenResult = GetToken().Result;
            string token = tokenResult.AccessToken;
            string user = tokenResult.Account.Username;
            // In case of shared mailbox access, replace the value of user variable with the UserPrincipalName or PrimarySmtpAddress of the shared mailbox. UserPrincipalName is recommended.
            string XOAUTH2 = Base64Encode($"user={user}\u0001auth=Bearer {token}\u0001\u0001");

            using (ExoPopClient client = new ExoPopClient())
            {
                // Receive the greating.
                var response = client.Receive();

                if (!response.StartsWith("+OK"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send("AUTH XOAUTH2");
                response = client.Receive();

                if (!response.StartsWith("+"))
                {
                    throw new Exception("OAuth is not supported");
                }

                client.Send(XOAUTH2);
                response = client.Receive();

                if (!response.StartsWith("+OK"))
                {
                    throw new Exception("Authentication failure");
                }

                // Download the first message.
                client.Send("STAT");
                response = client.Receive();

                if (!response.StartsWith("+OK"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send("LIST 1");
                response = client.Receive();

                if (!response.StartsWith("+OK"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send("QUIT");
                client.Receive();
            }

            Console.ReadLine();
        }

        static async Task<AuthenticationResult> GetToken()
        {
            IPublicClientApplication pca = PublicClientApplicationBuilder
                .Create(ClientId)
                .WithAuthority(Authority)
                .WithDefaultRedirectUri()
                .Build();

            var accounts = await pca.GetAccountsAsync();

            try
            {
                return await pca.AcquireTokenSilent(Scopes, accounts.FirstOrDefault()).ExecuteAsync();
            }
            catch (MsalUiRequiredException)
            {
                return await AcquireByDeviceCodeAsync(pca);
            }
        }

        private static async Task<AuthenticationResult> AcquireByDeviceCodeAsync(IPublicClientApplication pca)
        {
            try
            {
                var result = await pca.AcquireTokenWithDeviceCode(Scopes,
                    deviceCodeResult =>
                    {
                        Console.WriteLine(deviceCodeResult.Message);
                        return Task.FromResult(0);
                    }).ExecuteAsync();

                return result;
            }
            catch (Exception ex)
            {
                throw new Exception("Authentication failure", ex);
            }
        }

        private static string Base64Encode(string plainText)
        {
            var plainTextBytes = Encoding.UTF8.GetBytes(plainText);
            return Convert.ToBase64String(plainTextBytes);
        }
    }

    public class ExoPopClient : TcpClient
    {
        private Stream stream;
        private readonly string host = "outlook.office365.com";
        private byte[] receiveBuffer = new byte[1024];

        public ExoPopClient()
            : base("outlook.office365.com", 995)
        {
            stream = GetStream();

            var sslStream = new SslStream(stream, false, ValidateRemoteCertificate);
            sslStream.AuthenticateAsClient(host);
            stream = sslStream;
        }

        private static bool ValidateRemoteCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {
            return true;
        }

        public void Send(string command)
        {
            var commandBytes = Encoding.ASCII.GetBytes(command + Environment.NewLine);
            stream.Write(commandBytes, 0, commandBytes.Length);

            Console.WriteLine($"C:\t{command}");
        }

        public string Receive()
        {
            var stringBuilder = new StringBuilder();

            for (; ; )
            {
                var temp = stream.Read(receiveBuffer, 0, receiveBuffer.Length);
                stringBuilder.Append(Encoding.ASCII.GetString(receiveBuffer, 0, temp));

                if (0 < Available)
                {
                    continue;
                }

                if (2 <= stringBuilder.Length && stringBuilder[stringBuilder.Length - 2] == '\r' && stringBuilder[stringBuilder.Length - 1] == '\n')
                {
                    if (0 != Available)
                    {
                        stringBuilder.Append(Environment.NewLine);
                    }
                    else
                    {
                        break;
                    }
                }
            }

            var response = stringBuilder.ToString();

            Console.WriteLine($"S:\t{stringBuilder.Replace("\r\n", "\r\n\t").ToString(0, stringBuilder.Length - 1)}");

            return response;
        }
    }
}