Update to Exchange Online Reply-All Storm Protection

Last year we introduced the Reply-all Storm Protection feature to help protect your organization from unwanted reply-all storms. This update, coming soon, allows admins to customize various settings for it.

This message is associated with Microsoft 365 Roadmap ID 68840

When this will happen:

Roll-out for all Microsoft 365 environments will begin in early May and should be completed by early June.

How this will affect your organization:

This update gives Microsoft 365 tenant admins the ability to customize most all of the Reply-all Storm Protection settings to the following parameters:

Setting Value
Enabled/disabled  Enabled or Disabled
Minimum number of recipients 1000 to 5000 
Minimum number of reply-alls 5 to 20
Detection time (sliding window)  60 minutes (not customizable)
Block duration 1 to 12 hours

While most of the default settings remain the same, some have changed:

Setting Previous Default New Default
Enabled/disabled Enabled Enabled
Minimum number of recipients 5000   2500
Minimum number of reply-alls 10 10
Detection time (sliding window)  60 minutes 60 minutes (not customizable)
Block duration 4 hours  6 hours

What you need to do to prepare:

You can use the Remote Powershell Set-TransportConfig cmdlet to customize the settings:

There is nothing you need to do prepare for this update, but be aware that the change to the default minimum number of recipients from 5000 to 2500 may result in more reply-all storms being detected and acted upon than previously.

  • Set-TransportConfig -ReplyAllStormProtectionEnabled [$True:$False]
  • Set-TransportConfig -ReplyAllStormDetectionMinimumRecipients [1000 – 5000]
  • Set-TransportConfig -ReplyAllStormDetectionMinimumReplies [5 – 20]
  • Set-TransportConfig -ReplyAllStormBlockDuration [1 – 12]

Message ID: MC254418


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