"Reply all" email storms are somewhat amusing to watch from a distance due to the fact that they break the daily workflow mundaneness as people commonly inject jokes and other forms of humor into online exchanges. But they can also be a nuisance, especially in large organizations. There have been multiple reported instances where distribution lists containing hundreds of thousands of people have been part of an email chain where millions of emails have been exchanged for several hours.

To combat this problem, Microsoft released Reply-all Storm Protection back in 2020 for Microsoft 365 users utilizing Exchange Online. Today, the company has enabled new customization options for the feature.

Screenshot from Outlook

Reply-all Storm Protection automatically detects email storms for Microsoft 365 users and once that is done, automatically blocks reply-alls for a fixed period of time. While previously, some values required by the capability were hard-coded, Microsoft has made new customization options available today. It has also changed default values based on customer telemetry. You can view a comparison between the previous configuration and the modifications to it below.

Setting Previous Default New Default New Customization Options
Enabled/disabled Enabled Enabled Enabled or Disabled
Minimum number of recipients 5000 2500 1000 to 5000
Minimum number of reply-alls 10 10 5 to 20
Detection time sliding window 60 minutes 60 minutes 60 minutes (not customizable)
Block duration (once detected) 4 hours 6 hours 1 to 12 hours

The firm has also outlined commands for the Set-TransportConfig Remote PowerShell cmdlet that IT admins can use to configure these settings as per their organization's preference.

Microsoft has stated that the update is rolling out right now and should be available to all Microsoft 365 customers by mid-June. Future updates planned for the capability include insights, reports, and optional notifications. Depending upon feedback and interest, the company made also make these customization options available directly in the Exchange Admin Center.