The dead letter queue (DLQ) is a message's final landing spot, after automatic retries, after the APM adapter has attempted but failed to process a message.
There is a DLQ for every message topic.
You view and administer the DLQ contents differently, depending on whether your solution is Azure-hosted by Bentley or is on a server managed by your organization. This second type is called 'on-premise' or 'on prem', even if it is on your own choice of cloud server.
In the Azure environment, you should launch the Service Bus Explorer, and then expand the node "Topics" to find your desired message type.
An on prem DLQ is in your Rabbit MQ environment. You should log in here to view the DLQ counts and individual messages.