Published May 5, 2021

Exchange: Update to Exchange Online Reply-All Storm Protection

Reference ID: MC254418

MC254418 – Last year Microsoft 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:

SettingValue
Enabled/disabled Enabled or Disabled
Minimum number of recipients1000 to 5000 
Minimum number of reply-alls5 to 20
Detection time (sliding window) 60 minutes (not customizable)
Block duration1 to 12 hours

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

SettingPrevious DefaultNew Default
Enabled/disabledEnabledEnabled
Minimum number of recipients5000  2500
Minimum number of reply-alls1010
Detection time (sliding window) 60 minutes60 minutes (not customizable)
Block duration4 hours 6 hours

What you need to do to prepare:

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.

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

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

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