Microsoft launches a feature to block reply-all email threads

Microsoft announces “Reply All Storm Protection” for users of Microsoft Exchange Online and Microsoft Office 365. The feature is designed to prevent email storms, especially when hundreds or thousands of people start replying to an email thread.

“When a reply-all mail storm happens in your organisation, it can disrupt business continuity and in some cases even throttle the rest of your organisation’s email for a period of time,” wrote the Microsoft Exchange team in a blog post.

The new feature will mostly benefit large organisations, and is initially being rolled out to detect 10 reply-all emails to over 5,000 recipients within 60 minutes. It will then block subsequent replies to the thread for 4 hours.

Those who try to respond will see a message notifying them that their email was not sent because the conversation is too busy. The service will also suggest that users try replying to or forwarding the message to a smaller group of people rather than the entire chain.

“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,” wrote the Microsoft Exchange team.

Back in 1997, Microsoft had an infamous incident called “Bedlam DL3“, where around 25,000 people were on a distribution list and kept replying to the thread, generating 15,000,000 email messages and 195 gigabytes of data. The incident overwhelmed Microsoft’s own Exchange mail servers.

The new feature reportedly appears to be working for Microsoft’s own employees. They have mentioned that they’re already seeing the first version of the feature successfully reduce the impact of reply storms within Microsoft.

“Humans still behave like humans no matter which company they work for,” said the Microsoft Exchange team.

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