The Mailbox Check happens when a device sends a request to Iridium to get any messages that that may be waiting to be delivered to the device. A mailbox check may happen in either of 2 ways. First, every SBD report from the device to Iridium contains a 'Mailbox Check' request, these ones not charged for. The second way is for a specific 'Mailbox Check' request sent, at a set frequency, to Iridium. There is a setting within the firmware to set this 'On' and what the frequency for a mailbox checks. There is a cost to these, that can build up over even a short time, so we have set this to be 'Off' by default in all devices. You can turn it on in TP connect if you so desired, but we would not recommend it. 

All Iridium SBD devices such as the RockFLEET and the RockSTAR do not need the frequency setting to be set.

In the case of GSM devices, such as RockFLEET GSM or the RockAIR, which are mainly in cellular coverage, a Mailbox Check request via SBD messages are not being sent. And so a text or email might not be delivered, and Iridium only stores these for 24 hours, To get around this, TracPlus sends a notice to the device via GSM, to advise that there is a message waiting for delivery at Iridium. The device then sends a Mailbox Check to Iridium, to get the message delivered from there. This only happens with GSM devices that are mainly or totally within cellular coverage. The ones that you are seeing in the invoice are these and they are unlikely to a significant amount.