Gmail API Set Forwarding with Service Account

Question

Can anyone help as I'm getting bad request failed precondition errors when calling Gmail API to set forward address? Below is a C# .Net console app I'm trying do this with. I have delegated Domain Wide Authority to the Service Account.

Error:

Google.Apis.Requests.RequestError Bad Request [400] Errors [ Message[Bad Request] Location[ - ] Reason[failedPrecondition] Domain[global] ]

I think I was missing the User to impersonate. So, I added the user and now I get the following error.

Error:

Error:"unauthorized_client", Description:"Client is unauthorized to retrieve access tokens using this method.", Uri:""

namespace GmailForwarder
{
    class Program
    {

        static string ApplicationName = "GmailForwarder";

        static void Main(string[] args)
        {
            ServiceAccountCredential credential;
            string serviceAccount = "[email protected]";
            var certificate = new X509Certificate2(@"key.p12", "notasecret", X509KeyStorageFlags.Exportable);

            try
            {
                // Create credential
                credential = new ServiceAccountCredential(
                          new ServiceAccountCredential.Initializer(serviceAccount)
                          {
                            User = "[email protected]",
                            Scopes = new[] { GmailService.Scope.GmailSettingsSharing  }
                          }.FromCertificate(certificate));

                // Create Gmail API service.
                var service = new GmailService(new BaseClientService.Initializer()
                {
                    HttpClientInitializer = credential,
                    ApplicationName = ApplicationName,
                });

                string user = "[email protected]"; // test gmail account
                string fwdAddr = "[email protected]";

                ForwardingAddress fwdAddress = new ForwardingAddress();
                fwdAddress.ForwardingEmail = fwdAddr;

                var createFwdAddressResult = service.Users.Settings.ForwardingAddresses.Create(fwdAddress,"me").Execute();     

            }
            catch (Exception ex)
            {

            }     
        }
    }
} 

Answer 1

This worked for me when I used : https://mail.google.com/ and https://www.googleapis.com/auth/gmail.settings.sharing OAuth2 scopes