Email Settings API and OAuth2 for Apps Script: You are not authorized to access this API (Error 403)

Question

I understand that the Email Settings API is deprecated and will be turned down in Fall 2018. However, I need to create a script that delegates domain users (as Administrator) and the new Gmail API doesn't support the delegation feature yet.

I used OAuth2 library for Google Apps Script. I created a Service Account for the project. I delegated domain-wide delegation authority for the Service Account. I added the scope "https://apps-apis.google.com/a/feeds/emailsettings/2.0/" in Admin Console.

The OAuth2.0 code works. I have used it to successfully set a domain user's signature.

However, when I try to add delegate with Email Settings API the following code, I get this message:

Are you no longer able to access the deprecated Email Settings API? Any pointer would be appreciated. Thanks!

function addDelegate() {
  var email = "delegator@domain.com"
  var domainName = "domain.com"; 
  var username = email.split("@")[0];
  
  var authorizationScope = 'https://apps-apis.google.com/a/feeds/emailsettings/2.0/';
  
  var service = getDomainWideDelegationService("Email Settings API:", authorizationScope, email);

  if (!service.hasAccess()) {
    Logger.log("failed to authenticate as user " + email);
    Logger.log(service.getLastError());
  } else {
    Logger.log("successfully authenticated as user " + email);
  }
  
  var xml = '<?xml version="1.0" encoding="utf-8"?>' + 
            '<atom:entry xmlns:atom="http://www.w3.org/2005/Atom" xmlns:apps="http://schemas.google.com/apps/2006">' + 
              '<apps:property name="address" value="delegate@domain.com" />' + 
            '</atom:entry>';
       
  var base = "https://apps-apis.google.com/a/feeds/emailsettings/2.0/";

  var fetchArgs                = {};
  fetchArgs.headers            = {"Authorization": "Bearer " + service.getAccessToken()};
  fetchArgs.method             = "POST";
  fetchArgs.contentType        = "application/atom+xml";
  fetchArgs.payload            = xml;
  fetchArgs.muteHttpExceptions = true;
       
  var url = base + domainName + "/" + username + "/delegation";
  
  try {
    var response = UrlFetchApp.fetch(url, fetchArgs);     // Add delegate
    Logger.log("response = %s ", response.getContentText());
  } catch (e) {
    Logger.log("Add delegate failed: " + e);
  }
}