403 Запрещено при попытке использовать пакетную отправку Azure Notification Hub

Я пытаюсь использовать конечную точку API пакетной отправки для отправки ряда push-уведомлений конкретным пользователям в зависимости от их интересов. Мы пришли к выводу, что система тегов в центре уведомлений не обеспечивает необходимой нам гибкости.

У меня есть база данных, в которой хранятся токены устройств, которые могут использоваться конечной точкой прямой отправки, но даже использование стандартной информации, предоставленной Microsoft, дает ошибку 403 без информации о том, что пошло не так.

public class WnsNotificationService : BaseNotificationService
{
    private ISubscriptionProvider subscriptionProvider;

    /// <summary>
    /// Initializes a new instance of the <see cref="WnsNotificationService" /> class.
    /// </summary>
    /// <param name="telemetryService">The telemetry service.</param>
    /// <param name="subscriptionProvider">The subscription provider.</param>
    public WnsNotificationService(ITelemetryService telemetryService, ISubscriptionProvider subscriptionProvider) : base(telemetryService)
    {
        this.subscriptionProvider = subscriptionProvider;
    }

    /// <summary>
    /// Sends the new Airport Event notification to windows devices via WNS.
    /// </summary>
    /// <param name="airportEvent">The Airport Event.</param>
    /// <returns>
    /// Notification Result
    /// </returns>
    public override async Task<NotificationResult> SendNotification(AirportEvent airportEvent)
    {
        try
        {
            IEnumerable<IEnumerable<string>> deviceCollection =
                this.subscriptionProvider.GetSubscribedUserPNSHandles(airportEvent, PushNotificationPlatform.wns).Batch(1000);

            foreach (var devices in deviceCollection)
            {
                ServiceBusConnectionStringBuilder connectionString = new ServiceBusConnectionStringBuilder(ConfigurationManager.AppSettings["Microsoft.Azure.NotificationHubs.ConnectionString"]);
                string serviceBusNamespace = connectionString.Endpoints.First().Host;
                string namespaceKeyName = connectionString.SharedAccessKeyName;
                string namespaceKey = connectionString.SharedAccessKey;

                var uri = new Uri($"{ ConfigurationManager.AppSettings["Microsoft.Azure.NotificationHubs.Url"] }/{ ConfigurationManager.AppSettings["Microsoft.Azure.NotificationHubs.HubName"] }/messages/$batch?direct&api-version=2015-08");
                var request = WebRequest.CreateHttp(uri);
                request.Method = "POST";
                request.ContentType = @"multipart/mixed; boundary = ""simple-boundary""";
                request.Headers["Authorization"] = SharedAccessSignatureTokenProvider.GetSharedAccessSignature(namespaceKeyName, namespaceKey, serviceBusNamespace, TimeSpan.FromMinutes(45));
                request.Headers["ServiceBusNotification-Format"] = "windows";
                request.Headers["X-WNS-Type"] = "wns/raw";

                string body = this.GenerateBatchRequestBody(airportEvent.AirportEventId, devices);

                byte[] requestBytes = new ASCIIEncoding().GetBytes(body);
                Stream requestStream = request.GetRequestStream();
                requestStream.Write(requestBytes, 0, requestBytes.Length);

                request.GetResponse();
            }

            return new NotificationResult(true);
        }
        catch (Exception)
        {
            return new NotificationResult(false);
        }
    }

    /// <summary>
    /// Sends the new Airport Event notification to a specific windows device via WNS.
    /// </summary>
    /// <param name="deviceId">The device identifier.</param>
    /// <param name="airportEvent">The Airport Event.</param>
    /// <returns>
    /// Notification Result
    /// </returns>
    /// <exception cref="System.NotImplementedException">Not Implemented Exception</exception>
    public override Task<NotificationResult> SendNotificationToDevice(string deviceId, AirportEvent airportEvent)
    {
        throw new NotImplementedException();
    }

    /// <summary>
    /// Generates the batch request body.
    /// </summary>
    /// <param name="airportEventGuid">The airport event unique identifier.</param>
    /// <param name="devices">The devices.</param>
    /// <returns>Request Body</returns>
    private string GenerateBatchRequestBody(Guid airportEventGuid, IEnumerable<string> devices)
    {
        return @"
--simple-boundary
Content-type: text/xml
Content-Disposition: inline; name=notification

<toast><visual><binding template=""ToastText01""><text id=""1"">Hello from 
Batch Direct Send!</text></binding></visual></toast>
--simple-boundary
Content-type: application/json
Content-Disposition: inline; name=devices

['https://{foo}.notify.windows.com/?token={bar}']
--simple-boundary--";
    }
}

Для этого используется информация, предоставленная на https://azure.microsoft.com/en-us/blog/push-notification-hubs-batch-direct-send/