desus

That's exactly what they did. "The distribution list has been removed and associated emails are being traced and cleared." See http://support.nhs.net/servicestatus

That's exactly what they did do though. See http://support.nhs.net/servicestatus by 09:45 they'd disabled the distribution list and updated their service status page. There were that many emails in the system that at 15:30 people were still receiving emails sent at 09:30.

The domain is .nhs.net ;)

Some individual trusts have @trust.nhs.uk addresses, most - probably two thirds use the .nhs.net system.