C# 實作 SMTP 郵件發送完整元件教學

編輯：關於C語言

核心類別架構


namespace KSN.Web.Mail
{
    // 郵件內容實體
    public class MailMessage
    {
        public string Sender { get; set; }
        public StringCollection Receivers { get; } = new StringCollection();
        public string Subject { get; set; } = "";
        public string XMailer { get; set; } = "";
        public StringCollection Attachments { get; } = new StringCollection();
        public MailEncodings MailEncoding { get; set; } = MailEncodings.GB2312;
        public MailTypes MailType { get; set; } = MailTypes.Html;
        public byte[] MailBody { get; set; }
    }

    // 郵件編碼枚舉
    public enum MailEncodings { GB2312, ASCII, Unicode, UTF8 }

    // 郵件格式枚舉
    public enum MailTypes { Html, Text }

    // SMTP 驗證方式
    public enum SmtpValidateTypes
    {
        None,       // 無需驗證
        Login,      // AUTH LOGIN
        Plain,      // AUTH PLAIN
        CRAMMD5     // CRAM-MD5
    }

    // SMTP 發送核心類別
    public class KSN_Smtp
    {
        #region 欄位
        private TcpClient _tcpClient;
        private NetworkStream _networkStream;
        private StringDictionary _errorCodes = new StringDictionary();
        private StringDictionary _rightCodes = new StringDictionary();
        private string _errorMessage = "";
        private string _logs = "";
        private StringCollection _validateTypes = new StringCollection();
        private const string CRLF = "\r\n";
        private string _serverName = "smtp";
        private string _logPath = null;
        private string _userId = null;
        private string _password = null;
        private string _mailEncodingName = "GB2312";
        private bool _sendIsComplete = false;
        private SmtpValidateTypes _smtpValidateType = SmtpValidateTypes.Login;
        #endregion

        #region 屬性
        public string ErrorMessage => _errorMessage;
        public string LogPath { get => _logPath; set => _logPath = value; }
        public string UserID { get => _userId; set => _userId = value; }
        public string PassWord { get => _password; set => _password = value; }
        public SmtpValidateTypes SmtpValidateType { get => _smtpValidateType; set => _smtpValidateType = value; }
        #endregion

        #region 建構函式
        public KSN_Smtp(string server, int port)
        {
            _tcpClient = new TcpClient(server, port);
            _networkStream = _tcpClient.GetStream();
            _serverName = server;
            InitializeFields();
        }

        public KSN_Smtp(IPAddress ip, int port)
        {
            _tcpClient = new TcpClient(new IPEndPoint(ip, port));
            _networkStream = _tcpClient.GetStream();
            _serverName = ip.ToString();
            InitializeFields();
        }
        #endregion

        #region 核心方法
        private void InitializeFields()
        {
            _logs = "================" + DateTime.Now.ToLongDateString() + "  " + DateTime.Now.ToLongTimeString() + "================\r\n";

            // 錯誤狀態碼對照表
            _errorCodes.Add("421", "服務未就緒，關閉傳輸通道");
            _errorCodes.Add("432", "需要密碼轉換");
            _errorCodes.Add("450", "郵箱不可用");
            _errorCodes.Add("451", "放棄操作");
            _errorCodes.Add("452", "系統存儲不足");
            _errorCodes.Add("454", "暫時認證失敗");
            _errorCodes.Add("500", "郵箱地址錯誤");
            _errorCodes.Add("501", "參數格式錯誤");
            _errorCodes.Add("502", "命令不可實現");
            _errorCodes.Add("503", "命令次序不正確");
            _errorCodes.Add("504", "命令參數不可實現");
            _errorCodes.Add("530", "需要認證");
            _errorCodes.Add("534", "認證機制過於簡單");
            _errorCodes.Add("538", "需要加密認證");
            _errorCodes.Add("550", "郵箱不可用(未找到/不能用)");
            _errorCodes.Add("551", "用戶非本地");
            _errorCodes.Add("552", "存儲分配過量");
            _errorCodes.Add("553", "郵箱名不可用(格式錯誤)");
            _errorCodes.Add("554", "傳送失敗");
            _errorCodes.Add("535", "用戶身份驗證失敗");

            // 成功狀態碼對照表
            _rightCodes.Add("220", "服務就緒");
            _rightCodes.Add("221", "服務關閉傳輸通道");
            _rightCodes.Add("235", "驗證成功");
            _rightCodes.Add("250", "操作完成");
            _rightCodes.Add("251", "非本地用戶，將轉發");
            _rightCodes.Add("334", "服務器響應驗證 Base64 字串");
            _rightCodes.Add("354", "開始郵件輸入，以 <CRLF>.<CRLF> 結束");

            // 讀取伺服器歡迎訊息
            using (var reader = new StreamReader(_networkStream))
                _logs += reader.ReadLine() + CRLF;
        }

        private string SendCommand(string cmd, bool isMailData)
        {
            if (string.IsNullOrWhiteSpace(cmd)) return null;

            if (!isMailData) cmd += CRLF;
            _logs += cmd;

            byte[] cmdBytes;
            if (!isMailData)
            {
                cmdBytes = Encoding.ASCII.GetBytes(cmd);
                _networkStream.Write(cmdBytes, 0, cmdBytes.Length);
            }
            else
            {
                cmdBytes = Encoding.GetEncoding(_mailEncodingName).GetBytes(cmd);
                _networkStream.BeginWrite(cmdBytes, 0, cmdBytes.Length, AsyncCallback, null);
            }

            using (var reader = new StreamReader(_networkStream))
            {
                string response = reader.ReadLine();
                _logs += response + CRLF;

                string statusCode = response.Substring(0, 3);
                bool isKnown = false, isSuccess = true;

                foreach (string code in _errorCodes.Keys)
                    if (statusCode == code) { isKnown = true; isSuccess = false; break; }
                foreach (string code in _rightCodes.Keys)
                    if (statusCode == code) { isKnown = true; break; }

                if (!isKnown) SetError("非合法 SMTP 主機或服務器拒絕服務");
                else if (!isSuccess) SetError(statusCode + ":" + _errorCodes[statusCode]);
                else _errorMessage = "";

                return response;
            }
        }

        private void LoginByLogin()
        {
            string userB64 = ConvertToBase64(_userId, "ASCII");
            string passB64 = ConvertToBase64(_password, "ASCII");
            SendCommand("helo " + _serverName, false);
            SendCommand("auth login", false);
            SendCommand(userB64, false);
            SendCommand(passB64, false);
        }

        private void LoginByPlain()
        {
            string nullChar = ((char)0).ToString();
            string loginStr = nullChar + _userId + nullChar + _password;
            string b64 = ConvertToBase64(loginStr, "ASCII");
            SendCommand("helo " + _serverName, false);
            SendCommand(b64, false);
        }

        private void LoginByCramMd5()
        {
            SendCommand("helo " + _serverName, false);
            string response = SendCommand("auth CRAM-MD5", false);
            string challenge = response.Substring(4);
            // 注意：原始碼引用 KSN_MACTripleDES，此處略過具體 HMAC-MD5 實作
            // 實際應使用 HMACMD5 類別計算回應
        }

        public bool SendMail(MailMessage mail)
        {
            bool sent = true;
            try
            {
                // 驗證必要條件
                if (mail.Sender == null) SetError("未設置發件人");
                if (mail.Receivers.Count == 0) SetError("至少需要一個收件人");
                if (_smtpValidateType != SmtpValidateTypes.None && (string.IsNullOrEmpty(_userId) || string.IsNullOrEmpty(_password)))
                    SetError("需 SMTP 驗證但未提供帳號");

                // 登入
                switch (_smtpValidateType)
                {
                    case SmtpValidateTypes.None: SendCommand("helo " + _serverName, false); break;
                    case SmtpValidateTypes.Login: LoginByLogin(); break;
                    case SmtpValidateTypes.Plain: LoginByPlain(); break;
                    case SmtpValidateTypes.CRAMMD5: LoginByCramMd5(); break;
                }

                // MAIL FROM
                SendCommand("mail from:<" + mail.Sender + ">", false);

                // RCPT TO
                foreach (string rcpt in mail.Receivers)
                    SendCommand("rcpt to:<" + rcpt + ">", false);

                // DATA
                SendCommand("data", false);

                // 建構郵件內容
                _mailEncodingName = mail.MailEncoding.ToString();
                string message = BuildMailContent(mail);

                // 發送郵件資料
                SendCommand(message, true);
                if (_sendIsComplete) SendCommand("QUIT", false);
            }
            catch { sent = false; }
            finally
            {
                Disconnect();
                if (!string.IsNullOrEmpty(_logPath)) WriteLog();
            }
            return sent;
        }

        private string BuildMailContent(MailMessage mail)
        {
            string msg = "Subject:" + mail.Subject + CRLF;
            msg += "X-mailer:" + mail.XMailer + CRLF;
            msg += "MIME-Version:1.0" + CRLF;

            if (mail.Attachments.Count == 0)
            {
                if (mail.MailType == MailTypes.Text)
                {
                    msg += "Content-Type:text/plain;" + CRLF + " ".PadRight(8) + "charset=\"" + mail.MailEncoding + "\"" + CRLF;
                    msg += "Content-Transfer-Encoding:base64" + CRLF + CRLF;
                    if (mail.MailBody != null) msg += Convert.ToBase64String(mail.MailBody) + CRLF + CRLF + CRLF + "." + CRLF;
                }
                else // Html
                {
                    msg += "Content-Type:multipart/alternative;" + CRLF + " ".PadRight(8) + "boundary=\"=====003_Dragon310083331177_=====\"" + CRLF + CRLF + CRLF;
                    msg += "This is a multi-part message in MIME format" + CRLF + CRLF;
                    msg += "--=====003_Dragon310083331177_=====" + CRLF;
                    msg += "Content-Type:text/html;" + CRLF + " ".PadRight(8) + "charset=\"" + mail.MailEncoding.ToString().ToLower() + "\"" + CRLF;
                    msg += "Content-Transfer-Encoding:base64" + CRLF + CRLF;
                    if (mail.MailBody != null) msg += Convert.ToBase64String(mail.MailBody) + CRLF + CRLF;
                    msg += "--=====003_Dragon310083331177_=====--" + CRLF + CRLF + CRLF + "." + CRLF;
                }
            }
            else // 有附件
            {
                // 附件處理邏輯（略，原始碼完整包含 multipart/mixed 邊界處理）
                // 核心：各附件讀取檔案 → Base64 → 組裝 MIME 部分
            }
            return msg;
        }

        private void AsyncCallback(IAsyncResult ar) => _sendIsComplete = ar.IsCompleted;

        private void Disconnect()
        {
            try { _networkStream?.Close(); _tcpClient?.Close(); } catch { }
        }

        private void SetError(string msg)
        {
            _errorMessage = msg;
            _logs += msg + CRLF + "【郵件處理動作中止】" + CRLF;
            Disconnect();
            throw new ApplicationException(msg);
        }

        private string ConvertToBase64(string str, string encoding)
        {
            byte[] bytes = Encoding.GetEncoding(encoding).GetBytes(str);
            return Convert.ToBase64String(bytes);
        }

        private void WriteLog()
        {
            using (var fs = File.Exists(_logPath) ? new FileStream(_logPath, FileMode.Append, FileAccess.Write) : new FileStream(_logPath, FileMode.Create, FileAccess.Write))
            {
                byte[] logBytes = Encoding.GetEncoding("gb2312").GetBytes(CRLF + CRLF + _logs);
                fs.Write(logBytes, 0, logBytes.Length);
            }
        }
        #endregion
    }
}

使用範例


var smtp = new KSN_Smtp("smtp.gmail.com", 587)
{
    UserID = "your@gmail.com",
    PassWord = "your_password",
    SmtpValidateType = SmtpValidateTypes.Login,
    LogPath = @"c:\smtp_log.txt"
};

var mail = new MailMessage
{
    Sender = "your@gmail.com",
    Subject = "測試郵件",
    MailType = MailTypes.Html,
    MailEncoding = MailEncodings.UTF8,
    MailBody = Encoding.UTF8.GetBytes("<h1>Hello World</h1>")
};
mail.Receivers.Add("recipient@example.com");

bool success = smtp.SendMail(mail);
Console.WriteLine(success ? "發送成功" : "發送失敗: " + smtp.ErrorMessage);