[Scaevolus]

Page 5: "It emits an event, then immediately returns a response — meaning it always reports success (201), regardless of whether the downstream email handler succeeds or fails."

This is not the right way to accomplish this. If you want a Lambda function to trigger background processing, you invoke another lambda function before returning. Using Node.js events for background processing doesn't work, since the Lambda runtime shuts down the event loop as quickly as possible.