Microsoft Corp. is rolling out a new feature in Office 365 designed to block so-called “email storms” that include “reply-all” to a large number of people on a distribution list.

The Reply All Storm Protection feature is described by the Exchange Team in a blog post last week as mostly benefiting large organizations that have large distribution lists. When the feature detects a likely reply in an email storm taking place, it will block subsequent attempts to reply-all to the thread and return a non-delivery receipt to the sender.

At launch, the new feature is triggered when there are 10 reply-alls to more than 5,000 recipients within 60 minutes. Once triggered, replies are blocked on a reply-all thread for four hours.

“When a reply all mail storm happens in your organization, it can disrupt business continuity and in some cases even throttle the rest of your organization’s email for a period of time,” the Microsoft Exchange team said. “The Reply All Storm Protection feature will help reduce the impact that Reply All storms can have on large organizations.”

The term email storm has its own Wikipedia entry. Also called a “reply allpocalypse,” a sudden spike in reply-all messages on an email distribution list can cause issues including traffic than can render email servers inoperative. Severe email storms are described as being similar to a distributed denial-of-service attack.

Although the new feature is arguably welcomed, not all Microsoft customers will initially receive it. On launch, the new feature is only available to large enterprise users but will be rolled out to small to medium-sized businesses over time.

Microsoft also intends to add new features, including the ability to let administrators customize the email storm detection mechanism such as setting a reply-all number whereby the feature would be triggered.

Image: Microsoft