Microsoft Moves to End Email Reply-All Storms

Microsoft is now rolling out a feature it calls Reply All Storm Protection to Exchange Online, a solution to a problem that Microsoft, ironically, experiences regularly internally. The feature was first announced last November at Ignite 2019.

“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,” a post credited to the Exchange Transport Team reveals. “The Reply All Storm Protection feature [will] help reduce the impact that Reply All storms can have on large organizations. Today we’re pleased to announce that the feature is now rolling out in Office 365 worldwide.”

To be clear, the firm is referring to Office 365 commercial; this feature is for Exchange Online, not

Here’s how this new feature works: When Exchange Online detects a likely Reply All storm taking place in a large distribution list (DL), it will block subsequent Reply All attempts and will respond with a Non-Delivery Report (NDR) to the sender. Reply All Storm Protection detects a Reply All storm by looking for 10 Reply Alls to over 5000 recipients within 60 minutes. Subsequent replies to the thread are then blocked for 4 hours.

Over time, Microsoft will use telemetry and customer feedback to improve this feature and it expects to “tweak, fine-tune, and enhance Reply All Storm Protection” as a result of this feedback.

“We’re already seeing the first version of the feature successfully reduce the impact of Reply All storms within Microsoft (humans still behave like humans no matter which company they work for) and believe it will also benefit many other organizations as well,” the team notes.

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