I have two Series 2 XBees communicating in API mode and sending data every few seconds, repetitively. Each package has a unique number so I can track them. I am getting some strange behavior when they go out of range and come back in range that leads me to a question. When the receiving XBee send out a Transmit Status (let's call it an ACK), does it do anything to check the validity of the package received before sending the ACK to the transmitting XBee? For example, if the receiving XBee receives a package for which the checksum indicates a problem, would the receiving XBee still send an ACK to the transmitting XBee? (of course, the receiving XBee got something, albeit with errors.)