For what it is worth, when I did this earlier this year, I never received the email, but when I checked my account on recreation.gov, the reservation was listed, and there was a button to push that said something like "confirm reservation" or some such.
I had an overnight permit reservation. My understanding for that type of reservation (i.e., overnight) was that, if you don't confirm, they will release it at 10 a.m. your day of entry. If you do confirm, they will hold it until the end of the day.
Good luck!
