What you want is the quality of service "QoS" flag set at connection time (edit: sorry, correction, publish time): https://docs.oasis-open.org/mqtt/mqtt/v5.0/o

What you want is the quality of service "QoS" flag set at connection time (edit: sorry, correction, publish time): https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901103 https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.h... Bear in mind that there are always limits in how guaranteed a "guaranteed delivery" actually is. "Exactly once" means that the broker has received it and the client has acknowledged that. What happens beyond the broker is outside the scope of the protocol and would have to be handled at the application level, eg using transaction ids, etc.