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Microsoft Rolls Out Protection Against 'Reply All' Email Storms

Hitting reply all to thousands of recipients will no longer result in millions of emails taking a system offline.

 & Matthew Humphries Former Senior Editor

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One sure fire way to take down an email system is to send an email with thousands of recipients attached only to have a few of them hit reply all, generating millions more emails. Now Microsoft is introducing protection against what is commonly referred to as "reply all email storms."

As The Verge reports, the new feature is for Office 365 users and called Reply All Storm Protection. It works by adding a new type of monitoring to Microsoft's Exchange Online hosted email system which looks specifically for potential email storms. To begin with, one condition will be monitored: if 10 reply all responses to over 5,000 recipients are detected within a 60 minute period, subsequent replies to that email thread will be blocked for four hours.



By taking such action, recipients who typically respond with "please remove me from this thread" or some variation on that will no longer trigger a storm. Instead, Microsoft will just temporarily block communication in a bid to prevent an email system meltdown.

When the block does happen, Office 365 users will be informed that any reply they are trying to send "wasn't sent," with the reason being "the conversation is too busy with too many people." The feedback also suggests what action to take, including not attempting to resend the email and to consider sending an email to a smaller number of recipients.

Microsoft is going to look at usage telemetry and listen to customer feedback to "tweak, fine-tune, and enhance" the protection over time. The end goal is hopefully email storms being a thing of the past, not just for businesses, but for all email users. If Microsoft can guarantee they won't happen, it acts as another marketing tool to help sell Office 365 to organizations.

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About Our Expert

Matthew Humphries

Matthew Humphries

Former Senior Editor

My Experience

I started working at PCMag in November 2016, covering all areas of technology and video game news. Before that I spent nearly 15 years working at Geek.com as a writer and editor. I also spent the first six years after leaving university as a professional game designer working with Disney, Games Workshop, 20th Century Fox, and Vivendi.

I hold two degrees: a Bachelor's degree in Computer Science and a Master's degree in Games Development. My first book, Make Your Own Pixel Art, is available from all good book shops.

My Areas of Expertise

  • PC components and system building
  • Raspberry Pi
  • Software development
  • Storage technology
  • Video games and gaming hardware

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