I'm using the FEZ Panda board and the v4.1 Micro Framework (FEZ doesn't support v4.2 yet).
From the debugging I did, it looks like if the Coordinator node is off, then the frameBuffer in XBeeModule fills up as the frames in the buffer are not removed.
I'm not sending a high message volume, 2 - 3 messages every 5 seconds, each message approx 32 bytes in length.
I have a workaround at the moment, which is to call DropCallback() after SendString() in order to clear the frame from the buffer explicitly.
This also meant that I had to add a line of code to the XBeeModule.ReceivedTransmitStatus method to check if the sentFrame is already null.
ApiFrame sentFrame = frameBuffer[frameID];
if (sentFrame == null) return; // Added this line
A bit of a hack, but it means that the sensor nodes keep transmitting even when the Coordinator is powered down and recover when the Coordinator node re-appears on the network. Without this, the sensor nodes would require rebooting if the Coordinator node
ever failed for a period of time.