Updated A test email sent by accident to 850,000 NHS workers has caused utter chaos after being sent from an apparently incorrectly configured* email distribution list.

The sender, whom The Register will identify only as R, sent the blank message with a subject line that simply read "test" to a distribution list called CroydonPractices, according to irritated health service workers who contacted us.

The message somehow found its way to all NHS.net email addresses – and was immediately magnified by thoughtless people hitting "reply" to point out the error and demand they be removed.

Sources said actual work emails were delayed by at least three hours at the time of writing, thanks to the huge volumes of traffic snarling up NHS.net servers. By 11.30am we were told that 70 or 80 people had replied to the message, inadvertently copying it to all 850k NHS employee addresses.

The NHSmail service runs on Microsoft Exchange. A statement on its service status page said:

An issue with a distribution list has meant that several test emails have been widely received by users. This has been exacerbated by recipients replying in response and increasing the volume of emails associated with the list. The impact of this issue has meant that some users are unable to access OWA due to the volume of emails being circulated. The distribution list has been removed and associated emails are being traced and cleared.

R, whose signature block lists them as a senior associate ICT delivery facilitator at South East CSU, had understandably turned their mobile phone off by the time El Reg phoned to ask if they had realised the error.

The NHS phoned us to give us their statement on the snafu, pragmatically pointing out that an emailed statement might take some time to arrive.

An NHS Digital spokeswoman said: "Some users have experienced a short delay in the NHS mail system this morning. Action has been taken to resolve this issue."

They continued: "A number of email nets have been operating slower than normal due to an NHS mail user setting up an email distribution list which inadvertently included everyone on the NHS mail system. As soon as we became aware of the email we deleted the distribution list so that no one could respond to it. We anticipate the issue will be rectified very soon." ®

*Either a config mishap by the person who created the address caused all members, including the erroneously included NHS.net list recipients (rather than just the admin) to send out messages to the entire group or many or nearly all of the recipients are hitting "reply all". Although one can never underestimate a User, the former is more likely, given the apparent scale of the email storm.

Updated at 16.00: Correction - NHS Direct has been in touch to clarify that of their 1.2 million staffers, 850k are email address-havers. That's much better, right?