An email storm is a sudden spike of Reply All messages on an email

Email

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

 distribution list

Distribution list

Distribution list is a term sometimes used for a function of email clients where list of email addresses is used to email everyone on the list at once. This can be referred to as an electronic mailshot. It differs from a mailing list, electronic mailing list or the email option found in an Internet...

, usually caused by a controversial or misdirected message. Such storms start when multiple members of the distribution list reply to the entire list at the same time in response to the instigating message. Other members soon respond, usually adding vitriol to the discussion, asking to be removed from the list, or pleading for the cessation of messages. If enough members reply to these unwanted messages this triggers a chain reaction

Chain reaction

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....

 of email messages. The sheer load of traffic generated by these storms can render the email servers

Mail transfer agent

Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...

 inoperative, similar to a DDoS attack

Denial-of-service attack

A denial-of-service attack or distributed denial-of-service attack is an attempt to make a computer resource unavailable to its intended users...

.

Some email viruses

Computer virus

A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...

 also have the capacity to create email storms, by sending copies of themselves to an infected user's contacts, including distribution lists, infecting the contacts in turn.

Incidents

• On October 14, 1997, a Microsoft
  Microsoft
  Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
  
   employee noticed that they were on an as-yet unknown email distribution list 'Bedlam DL3', and emailed the list asking to be removed. This list contained approximately a quarter of the company's employees, 13,000 email addresses. Other users replied to the list with similar requests and still others responded with pleas to stop replying to the list. A Microsoft employee estimates that 15 million emails were sent, using 195 GB of bandwidth.
• On October 3, 2007, an email storm was generated at the U.S. Department of Homeland Security, causing more than 2.2 million messages to be sent, and exposing the names of hundreds of security professionals.
• U.S. State Department employees were warned they could face disciplinary action for taking part in a massive email storm that "nearly knocked out one of the State Department's main electronic communications systems."