Reply All Storm Protection in Exchange Online
Published May 07 2020 11:00 AM 128K Views

Update 5/5/2021: For additional information on this subject and new settings, please see this post.

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. At Microsoft Ignite 2019 we announced the Reply All Storm Protection feature to help reduce the impact that reply all storms can have on large organizations. This feature has completed our rollout, and is now available to all Office 365 tenants worldwide (except GCC-High… it’s coming).

Initially the Reply All Storm Protection feature will mostly benefit large organizations who have large distribution lists. When the feature detects a likely reply all storm taking place on a large DL it will block subsequent attempts to reply all to the thread and will return an NDR to the sender. The reply all block will remain in place for several hours.

Initially the Reply All Storm Protection feature will use the following conditions to detect a reply all storm:

  • 10 reply all-s to over 5000 recipients within 60 minutes

It will then block subsequent replies to the thread for 4 hours.

The reply all storm NDR will look something like this:



Over time, as we gather usage telemetry and customer feedback, we expect to tweak, fine-tune, and enhance the Reply All Storm Protection feature to make it even more valuable to a broader range of Office 365 customers.

We’d love to hear your feedback on this feature. We are considering possible future enhancements to improve our reply all storm detection accuracy, add admin customizable thresholds and block duration, as well as producing reply all storm reports and notifications.

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 Exchange Transport Team

Version history
Last update:
‎May 05 2021 12:32 PM
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