Send mail through SMTP using oauth 2

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.NET

API

MVC5

google-api

This one is the error which i am getting every time :

"Error:"unauthorized_client", Description:"Client is unauthorized to retrieve access tokens using this method, or client not authorized for any of the scopes requested.", Uri:"

Code :

var certificate = new X509Certificate2(Server.MapPath("folder/serviceaccountgenratedcredecialfile.p12"), "notasecret", X509KeyStorageFlags.Exportable);
         var credential = new ServiceAccountCredential(new ServiceAccountCredential
         .Initializer("service-account-email@my-project-name.iam.gserviceaccount.com")
         {

             Scopes = new[] { "https://mail.google.com/" },
             User = "user-email@gmail.com"
         }.FromCertificate(certificate));

         bool result =  credential.RequestAccessTokenAsync(CancellationToken.None).Result;

What I have tried:

I have tried every possible solution which is available on google.