Given the task definition and input, reply with output. Generate an appropriate single-sentence summary for the given text such that it includes the main topic of the text.

One of the health system's employees fired off the message on Monday morning without realising they had copied in 840,000 of their colleagues. The action quickly clogged up the system and was exacerbated by users hitting "reply all" to complain. The distribution list was disabled at 10:00 GMT, but some users continue to have problems. The secure email system is used by NHS staff and other approved organisations to discuss healthcare and related activities. "It's driving me bananas," one doctor - who asked not to be identified - told the BBC. "The thing is hundreds of people have been replying to all. "My NHS email is very important to me because it's the only secure way I can send and receive anything safely about my patients. "So, this is a major problem [and] potentially a risk to patients." A spokeswoman for NHS Digital said it was not a member of its IT team who had sent the message, but declined to identify the culprit, saying they were not to blame. "A number of email accounts have been operating slowly," said NHS Digital in a statement. "This was due to an NHS Mail user setting up an email distribution list which, because of a bug in the supplier's system, inadvertently included everyone on the NHS Mail list. "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 the issue will be rectified very soon."

An email that was accidentally sent to all the NHS's staff in England has caused havoc.