title: Client is unauthorized to retrieve access tokens using this method Gmail API C#
body: <p>I am getting the following error when i tried to authorize gmail api using service account</p>
  
  <blockquote>
    <p>"Client is unauthorized to retrieve access tokens using this method"</p>
  </blockquote>
  
  <pre><code>static async Task MainAsync()
      {
  
          sstageEntities db = new sstageEntities();
          //UserCredential credential;
          Dictionary&lt;string, string&gt; dictionary = new Dictionary&lt;string, string&gt;();    
  String serviceAccountEmail =
  "xxx.iam.gserviceaccount.com";
  
          var certificate = new X509Certificate2(
              AppDomain.CurrentDomain.BaseDirectory +
                "xxx-8c7a4169631a.p12",
              "notasecret",
              X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.Exportable);
  
          //string userEmail = "user@domainhere.com.au";
  
          ServiceAccountCredential credential = new ServiceAccountCredential(
              new ServiceAccountCredential.Initializer(serviceAccountEmail)
              {
                  User = "xxx@xxx.com",
                  Scopes = new[] { "https://mail.google.com/" }
              }.FromCertificate(certificate)
          );
  
  
          // Create Gmail API service.
          var gmailService = new GmailService(new BaseClientService.Initializer()
          {
              HttpClientInitializer = credential,
              ApplicationName = ApplicationName,
          });
  
          // Define parameters of request.
  
          var emailListRequest = gmailService.Users.Messages.List("xxx@xxx.com");
          emailListRequest.LabelIds = "INBOX";
          emailListRequest.IncludeSpamTrash = true;
          emailListRequest.Q = "from:bpm@xxx.co.in is:unread";
  
  
  
          //Get our emails
          var emailListResponse = await emailListRequest.ExecuteAsync();
  </code></pre>
  
  <p>I am using the p12 key which i got while creating service account.But when i run my console app the following error occurs.Any help would be really appreciated.</p>
  
  <p>Thanks in advance !</p>