No, they will not rebook based on a mail. As they know they would need to collect the FOP to issue the ticket. They will write back either "ok, let's do it - call in" or "No, can do!"... or might just send the points your way.

But I have seen it now MAAAAAANY times that even though the agent promised the seats will go back into the inventory, they don't! Even worse, they are taken out as soon as you do a reservation... no matter if that gets ticketed or not. Got burned by that a few times. I asked ok, can we do the folloing itineary and if I then decide for a different connection, will the inventory be available again.... and always heard "yes, will be released back to the pool, once the reservation hold is released". But NEVER happened. The seats were always gone for good. So NEVER do a dummy booking over the phone to see if a routing will work out if you are not prepared to then issue tickets. Even over the web, sometimes when the reservation system grabs the seats and you change your mind during check-out... the seats will not go back to the award inventory.

I think what they learn is: Inventory which gets released from a reservation go back to the pool. But not if they will go back to the same booking class and therefore e.g. the award pool.