Design a system to handle user notifications in a microservices architecture. How would you approach message queuing, delivery guarantees, and scaling the notification service?

``` I would utilize a message broker like RabbitMQ or Apache Kafka to decouple the notification service from other services, ensuring delivery guarantees through appropriate acknowledgment mechanisms, while scaling the service using horizontal scaling strategies along with topic partitions if using Kafka. Scenario Pressure: Discuss how you would handle spikes in message traffic and the implications for delivery times. Depth Trigger: Delve into the complexities of handling failures in the notification delivery and managing retry strategies. ```