Are you serious?
In Exchange, you can lock down a list to only allow specified senders. It takes about 5 sec, via the GUI or Powershell. We generally control these via another group -e.g. $list-senders are the only ones allowed to send to specified list. Even if one of the permitted senders screws up, the Reply-Alls don't go far.
If you have any smarts, and you're actually allowing end-users to set up email lists, you'd run some kind of script on a schedule to check for email-enabled groups with (recursive!) members > $number and verify that all of those have sender restrictions on them.
For the NHS, the fact the storm went on for that long is appalling - it should have taken approx. 2 mins to lock the list (assuming someone had to logon to a box to set the restriction). Give it 15 mins for someone to verbally raise the alarm... (although, again, if end-users can set up the lists, you'd expect some pretty gnarly monitoring to be in place to actually raise an alert itself, even just seeing if the queues are filling up.)
Biting the hand that feeds IT
