What happen, inside the XBee S2 module, after a Network ACK Failure (ACK timeout)?
I got a answer from the Digi Forum:

The network ack failure indicates that the packet failed to be received by the recipient within the specified time frame. This could be the result of several reasons including but not limited to:


module out of range
module no longer in the network
Broken link resulting in a node discovery to be required but do to time constraints, it was not able to occur
Timeout set to short to allow enough time for the packet to reach the recipient due to the number of hops required.


As far as the radio is concerned, once the failure occurs, the status frame is generated and issued out the UART (API mode only). At that point, nothing more will occur on the originator even if the packet is received later.