Sending an email to everyone in the company is usually a guaranteed way of making yourself unpopular, but the potential for annoyance is even greater if you have 1.2 million colleagues.

On Monday, NHS staff complained on Twitter about a “test email” sent by an IT contractor at Croydon NHS to everyone in the organisation, as well as replies to all in response to the message, leading to claims that the entire email system had crashed.

One health service statistician estimated that at least 186m emails, including replies to all asking to be taken off the distribution list, had been sent, clogging up people’s inboxes. NHS digital said 840,000 accounts were affected.

Another waste of a working day due to some pillick in #nhsmail sending email to entire directory, unable to connect and do my job #epicfail — Gavin (@68_gavin) November 14, 2016

Hmmmm... wonder if this is why I can't log in to #NHSmail at the moment ?! https://t.co/wBvuREsB6X — James Andrews (@aptaim) November 14, 2016

If you're trying to get in touch with us today, please be patient, it seems #NHSmail has gone down for the time being. We'll reply ASAP. A — Project iHypE (@PIhype) November 14, 2016

Slow handclap for the individual that sent a test email to the entire NHSMail user base, and bravo to those that "replied to all"... pic.twitter.com/zkg5uG7t2M — Colin McDonnell (@Malignanthero) November 14, 2016

#nhsmail 1.2 million people have received approx 151 emails in error this morning. That's 186 million needless emails so far today. — Graham Hyde (@GrahamHyde) November 14, 2016

A message to NHSmail users described the global email as a “high severity service incident”. It 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 [Outlook web access] due to the volume of emails being circulated. The distribution list has been removed and associated emails are being traced and cleared. In the meantime, users will experience slow performance with OWA and email delivery delays from internal and external sources to nhs.net addresses.”

In a statement dictated over the phone due to the problems with the email system, an NHS Digital spokeswoman said: “Some users have experienced short delays in the NHSmail system this morning. Action has been taken to resolve this issue.

“A number of email accounts have been operating slower than normal due to an NHSmail user setting up an email distribution list which inadvertently included everyone on the NHSmail system. As soon as we became aware of the issue, we deleted the distribution list so that no one else could respond to it. We anticipate that the issue will be rectified very soon.”

Complaints to the NHSmail helpdesk have trebled since the email system was introduced in May. The secure email service is approved by the Department of Health for sharing patient-identifiable and sensitive information.