为什么Gmail API不能从C#发送超文本标记语言形式的电子邮件？

using Google.Apis.Gmail.v1;
using Google.Apis.Gmail.v1.Data;
using Google.Apis.Auth.OAuth2;
using Google.Apis.Services;
using System;
using System.IO;
using System.Threading;

class Program
{
    static void Main(string[] args)
    {
        // 认证和授权
        UserCredential credential;
        using (var stream = new FileStream("credentials.json", FileMode.Open, FileAccess.Read))
        {
            string credPath = "token.json";
            credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
                GoogleClientSecrets.Load(stream).Secrets,
                new[] { GmailService.Scope.GmailSend },
                "user",
                CancellationToken.None,
                new FileDataStore(credPath, true)).Result;
        }

        // 创建Gmail API服务
        var service = new GmailService(new BaseClientService.Initializer()
        {
            HttpClientInitializer = credential,
            ApplicationName = "Gmail API C#",
        });

        // 构建邮件消息
        var message = new Message();
        message.Raw = Base64UrlEncode(CreateMessageWithEmail(CreateEmailMessage()));

        // 发送邮件
        service.Users.Messages.Send(message, "me").Execute();
    }

    // 创建邮件消息
    private static Google.Apis.Gmail.v1.Data.Message CreateEmailMessage()
    {
        var emailMessage = new Google.Apis.Gmail.v1.Data.Message();
        emailMessage.Subject = "Hello from Gmail API";
        emailMessage.From = new Google.Apis.Gmail.v1.Data.MessagePartHeader()
        {
            Name = "Sender Name",
            Email = "sender@example.com"
        };
        emailMessage.To = new List<Google.Apis.Gmail.v1.Data.MessagePartHeader>()
        {
            new Google.Apis.Gmail.v1.Data.MessagePartHeader()
            {
                Name = "Recipient Name",
                Email = "recipient@example.com"
            }
        };

        // 设置HTML内容
        string htmlContent = "<h1>This is a test email</h1><p>Hello, <b>World!</b></p>";
        emailMessage.Body = new Google.Apis.Gmail.v1.Data.MessagePart()
        {
            MimeType = "text/html",
            Body = Base64UrlEncode(htmlContent)
        };

        return emailMessage;
    }

    // 创建包含邮件消息的RFC 2822格式消息
    private static Google.Apis.Gmail.v1.Data.Message CreateMessageWithEmail(Google.Apis.Gmail.v1.Data.Message emailMessage)
    {
        var buffer = Encoding.UTF8.GetBytes(emailMessage.ToJson());
        var base64 = Convert.ToBase64String(buffer);
        return new Google.Apis.Gmail.v1.Data.Message() { Raw = base64 };
    }

    // Base64 URL编码
    private static string Base64UrlEncode(string input)
    {
        var inputBytes = Encoding.UTF8.GetBytes(input);
        return Convert.ToBase64String(inputBytes)
            .Replace('+', '-')
            .Replace('/', '_')
            .Replace("=", "");
    }
}