This feature was not originally available because with persistence enabled, messages could be stored locally without ever knowing if they could be sent. The client application could have created the client with an erroneous broker address or port for instance.

To enable messages to be published when the application is disconnected MQTTAsync_createWithOptions must be used instead of MQTTAsync_create to create the client object. The MQTTAsync_createOptions field sendWhileDisconnected must be set to non-zero, and the maxBufferedMessages field set as required - the default being 100.

MQTTAsync_getPendingTokens can be called to return the ids of the messages waiting to be sent, or for which the sending process has not completed.