Client Credential Flow を使って Exchange Online に SMTP で接続する

using Microsoft.Identity.Client;
using System.Net.Security;
using System.Net.Sockets;
using System.Net;
using System.Security.Cryptography.X509Certificates;
using System.Text;

namespace SmtpOAuthCcfDemo
{
    internal class Program
    {
        private const string ClientId = "<控えておいたアプリケーション (クライアント) ID>";
        private const string Secret = "<控えておいたクライアント シークレット>";
        private const string TenantId = "<接続先のテナント名 (contoso.onmicrosoft.com など)>";
        private static readonly string[] Scopes = new string[] { "https://outlook.office365.com/.default" };

        private const string SenderSmtpAddress = "<差出人の SMTP アドレス (アプリがフル アクセス権を与えられているメールボックス)>";
        private const string RecipientSmtpAddress = "<宛先の SMTP アドレス>";

        static void Main(string[] args)
        {
            var tokenResult = GetToken().Result;
            string token = tokenResult.AccessToken;
            string XOAUTH2 = Base64Encode($"user={SenderSmtpAddress}\u0001auth=Bearer {token}\u0001\u0001");

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

                if (!response.StartsWith("220"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send($"EHLO {Dns.GetHostName()}");
                response = client.Receive();

                if (!response.Contains("STARTTLS"))
                {
                    throw new Exception("STARTTLS is not supported");
                }

                client.Send("STARTTLS");
                response = client.Receive();

                if (!response.StartsWith("220"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.UpgradeToSsl();

                client.Send($"EHLO {Dns.GetHostName()}");
                response = client.Receive();

                if (!response.Contains("XOAUTH2"))
                {
                    throw new Exception("OAuth is not supported");
                }

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

                if (!response.StartsWith("334"))
                {
                    throw new Exception("Unexpected response received.");
                }

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

                if (!response.StartsWith("235"))
                {
                    throw new Exception("Authentication failure");
                }

                client.Send($"MAIL FROM: {SenderSmtpAddress}");
                response = client.Receive();

                if (!response.StartsWith("250"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send($"RCPT TO: {RecipientSmtpAddress}");
                response = client.Receive();

                if (!response.StartsWith("250"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send("DATA");
                response = client.Receive();

                if (!response.StartsWith("354"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send($"From: {SenderSmtpAddress}");
                client.Send($"To: {RecipientSmtpAddress}");
                client.Send("Subject: Test Mail");
                client.Send("");
                client.Send("Test message.");
                client.Send(".");

                response = client.Receive();

                if (!response.StartsWith("250"))
                {
                    throw new Exception("Unexpected response received.");
                }

                client.Send("QUIT");
            }

            Console.ReadLine();
        }

        static async Task<AuthenticationResult> GetToken()
        {
            var cca = ConfidentialClientApplicationBuilder
                .Create(ClientId)
                .WithClientSecret(Secret)
                .Build();

            return await cca.AcquireTokenForClient(Scopes)
                .WithAuthority(AzureCloudInstance.AzurePublic, TenantId)
                .ExecuteAsync();
        }

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

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

        public ExoSmtpClient()
            : base("outlook.office365.com", 587)
        {
            stream = GetStream();
        }

        public void UpgradeToSsl()
        {
            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')
                {
                    break;
                }
            }

            var response = stringBuilder.ToString();

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

            return response;
        }
    }
}