If a customer's API was down, the event would go back on the queue with a header saying to retry it after some time. You can do some sort of incantation to specifically retrieve messages with a suitable header value, to find messages which are ready to retry. We used exponential backoff, capped at one day, because the API might be down for a week.
I didn't think of RabbitMQ as a database when I started that work, but it looked a lot like it by the time I finished.
