Hi @Reinski,
The maximum amount of memory that can be used for QoS=0 queuing is set to 1/4 of the available memory. You can increase it by increasing the memory of your machine.
Please note that HiveMQ does only queue QoS=0 in-memory as a courtesy in the case that the receiver’s TCP socket is not writable. According to the MQTT specification it would also be correct to simply drop the messages without any queuing.
It seems to me that the root cause for the dropped messages you observe are subscribers that are not consuming messages quickly enough or not as quickly as messages are being published to your broker.
I hope this helps.
Kind regards,
Michael from the HiveMQ Team