The schedule can be changed to launch at a different time than usual, and it will still process. If the email is a standard launch and is already scheduled you can demote the email and then publish again with the new launch time. From the Scheduled tab click the email name and select demote.
If the email is a DOD automated email you will need to drop the file for processing before the scheduled outage. If the file drops during the outage the file will be held and processed when back online.