In message queueing a dead letter queue (DLQ), or dead letter topic (DLT) in some messaging systems, is a service implementation to store messages that the messaging system cannot or should not deliver. Although implementation-specific, messages can be routed to the DLQ for the following reasons:The message is sent to a queue that does not exist. The maximum queue length is exceeded. The message exceeds the size limit. The message expires because it reached the TTL The message is rejected by another queue exchange. The message has been read and rejected too many times.