I wonder how many emails per day the system usually handles; I would say I probably receive on the order of 100 emails a day (although most of those are automated messages) and probably send half a dozen, and I'm definitely on the low end.

It's been quite a few years since I was last an Exchange administrator, but I seem to recall that you could secure a distribution list such that only authorized members could send mail to it. Perhaps (hopefully) only a relatively small group at NHS are allowed to mail all 1.2M mailboxes, and within that group there are 120 very silly people.

I wonder if most employees lack the needed permission to send an e-mail to 1.2 million recipients. It seems like a smart thing to restrict that kind of action, precisely to avoid havoc.

Another possibility is that dealing with messages that have 1.2 million recipients so overloads the system that it it has only been able to deliver 120 of them so far.