An e-mail storm is a situation where people start hitting “reply all” on e-mail messages sent to a group, causing a dramatic uptick in the number of messages handled by a server and leaving individuals with in-boxes laden with messages. While an e-mail storm can be a topic of humor or irritation, it can also potentially pose a problem, as servers may become so overloaded that the deluge of e-mail effectively creates a denial of service attack, shutting down the server. For this reason, steps are usually taken to prevent e-mail storms.
Most commonly, an e-mail storm is associated with a listserv, a group of people signed up to an e-mail service. People can send messages out to everyone on the service with important information or correspondence, and individuals on the list can read these messages, reply to the sender, or choose to reply to all, sending a message to everyone on the list. People may hit “reply all” by accident and trigger an e-mail storm as infuriated subscribers respond to inform them that they didn't need to send the message to everyone, setting off a firestorm of e-mails.
This can also occur when a controversial matter comes up and everyone on the service wants to weigh in. Rather than sending messages to individual people and carrying on a private conversation, people start replying to everyone, creating an e-mail storm. People not interested in the discussion may send messages to everyone as well, pleading with people to stop e-mailing the entire service. When a service includes hundreds or thousands of subscribers, the server can quickly back up.
During an e-mail storm, handling of other messages on the server can slow to a crawl. Individual people caught up in the storm can have trouble accessing their inboxes, and it may take a long time for messages to load. Messages not related to the storm may be hidden under a pile of new messages from the e-mail storm, making it difficult for people to find correspondence they need to see and respond to.
In 2009, an e-mail storm at the US State Department actually resulted in threats of disciplinary action, with officials warning that the storm compromised servers and potentially exposed the names of officials working confidentially. Workers were warned to use the “reply all” option with care, only directing messages to an entire group of people on an e-mail distribution list when it was actually necessary.