Dead Letter Queues is a feature of MSMQ that tracks messages that are undeliverable, deleted, expired etc. NServiceBus endpoints will by default enable DLQ for all outgoing messages except messages that have a Time To Be Received(TTBR) set. This avoids expired messages ending up either in the TDLQ or DLQ (if non-transactional queues are used) wasting disk space on the machine when they time out.
DLQ can be disabled for the entire endpoint using the MSMQ connection string.
DLQ is be enabled by default for TTBR messages.