C＃Gmail API发送电子邮件：错误请求[400]

Question

我尝试发送基于Gmail API的电子邮件。我在Google Develover Console中创建了一个项目。 我提到了GmailService实例：https://developers.google.com/gmail/api/quickstart/dotnet

以下是发送电子邮件的代码：

var msg = new AE.Net.Mail.MailMessage { Subject = "Your Subject", Body = "Hello, World, from Gmail API!", From = new MailAddress("[you]@qq.com") };  msg.To.Add(new MailAddress("yourbuddy@qq.com")); 
msg.ReplyTo.Add(msg.From); // Bounces without this!! 
var msgStr = new StringWriter();  
msg.Save(msgStr); 

string[] Scopes = { GmailService.Scope.GmailSend }; 
string ApplicationName = "Gmail API .NET Quickstart"; 
UserCredential credential

using (var stream = new FileStream("client_secret.json", FileMode.Open, FileAccess.Read)) 
{ 
string credPath = System.Environment.GetFolderPath( System.Environment.SpecialFolder.Personal); 
credPath = Path.Combine(credPath, ".credentials/gmail-dotnet-quickstart.json"); 

credential = GoogleWebAuthorizationBroker.AuthorizeAsync( GoogleClientSecrets.Load(stream).Secrets, Scopes, "user", CancellationToken.None, new FileDataStore(credPath, true)).Result; 

Console.WriteLine("Credential file saved to: " + credPath); 
}

var gmail = new GmailService(new Google.Apis.Services.BaseClientService.Initializer() { HttpClientInitializer = credential, ApplicationName = ApplicationName, }); 
var result = gmail.Users.Messages.Send(new Google.Apis.Gmail.v1.Data.Message { Raw = Base64UrlEncode(msgStr.ToString()) }, "user").Execute();

但我只是得到了这个：

  

类型＆＃39; Google.GoogleApiException＆＃39;未处理的例外情况发生在   Google.Apis.dll其他信息：   Google.Apis.Requests.RequestError错误请求[400]   错误[     消息[错误请求]位置[ - ]原因[failedPrecondition]   域[全球]   ]

知道出了什么问题吗？谢谢！