An e-mail storm is a sudden spike of Reply All messages on an email distribution list, 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 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 of email messages. The sheer load of traffic generated by these storms can render the email servers carrying them inoperative, similar to a DDoS attack.

Some email viruses 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.

Specific incidents

• On October 14, 1997, a Microsoft 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 US Department of Homeland Security, causing more than 2.2 million messages to be sent, and exposing the names of hundreds of security professionals.
• 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."