C#检测邮箱是否真实存在

public async Task<bool> VerifyEmailAddressAsync(string emailAddress)
{
    string smtpServer = "mx1.qq.com"; 
    int port = 25;

    try
    {
        // 1. 使用 using 确保资源释放
        using (TcpClient tClient = new TcpClient())
        {
            // 异步连接，防止界面卡死，设置超时
            var connectTask = tClient.ConnectAsync(smtpServer, port);
            if (await Task.WhenAny(connectTask, Task.Delay(5000)) != connectTask)
            {
                // 连接超时
                return false; 
            }

            using (NetworkStream netStream = tClient.GetStream())
            using (StreamReader reader = new StreamReader(netStream, Encoding.ASCII))
            {
                // 读取服务器的初始欢迎信息 (例如: 220 qq.com Anti-spam GT for Coremail System)
                string response = await reader.ReadLineAsync();
                if (!CheckResponse(response, "220")) return false;

                // 2. 发送 HELO/EHLO
                await SendCommand(netStream, "HELO local.test");
                response = await reader.ReadLineAsync();
                if (!CheckResponse(response, "250")) return false;

                // 3. 发送 MAIL FROM (使用一个看起来合法的发件人)
                await SendCommand(netStream, "MAIL FROM:<check@test.com>");
                response = await reader.ReadLineAsync();
                if (!CheckResponse(response, "250")) return false;

                // 4. 发送 RCPT TO (关键步骤)
                await SendCommand(netStream, $"RCPT TO:<{emailAddress}>");
                response = await reader.ReadLineAsync();

                // 5. 判断结果
                bool isValid = false;
                if (CheckResponse(response, "250"))
                {
                    isValid = true; // 邮箱存在
                }
                else if (CheckResponse(response, "550"))
                {
                    isValid = false; // 邮箱不存在
                }
                
                // 6. 正常退出
                await SendCommand(netStream, "QUIT");
                
                return isValid;
            }
        }
    }
    catch (Exception ex)
    {
        // 处理网络错误等异常
        Console.WriteLine($"Error: {ex.Message}");
        return false;
    }
}

// 辅助方法：发送命令
private async Task SendCommand(NetworkStream stream, string command)
{
    byte[] data = Encoding.ASCII.GetBytes(command + "\r\n");
    await stream.WriteAsync(data, 0, data.Length);
}

// 辅助方法：检查响应代码
private bool CheckResponse(string response, string expectedCode)
{
    if (string.IsNullOrEmpty(response)) return false;
    // SMTP 响应格式通常是 "CODE Description"，例如 "250 OK"
    return response.StartsWith(expectedCode);
}