Microsoft rolls out protection against Office 365 email storms

guest · May 9, 2020

Microsoft rolls out protection against Office 365 email storms
May 8, 2020
https://www.bleepingcomputer.com/ne...t-protection-against-office-365-email-storms/

Microsoft is rolling out protection against Office 365 Reply-All email storms, an issue impacting Exchange Online users who are members of large and improperly locked down mail distribution lists.

Reply-All storms (also known as reply-allpocalypses) are huge chain reaction sequences of emails that start when a member of a large organization's email distribution list send a reply to all its members using the "Reply All" feature, in some cases effectively pinging the inboxes of thousands of people with a single mouse click.

Things get even more problematic when others use the "Reply All" feature, with tens if not hundreds of thousands of "email bombs" being delivered to all mailing list members for hours on end.
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New email storm protection blocks new replies

"When the feature detects a likely reply all storm taking place on a large DL it will block subsequent attempts to reply all to the thread and will return an NDR to the sender," the Microsoft Exchange team now announced. "The reply all block will remain in place for several hours."
The Reply All Storm Protection feature will send the bounce message — also known as a Non-Delivery Receipt (NDR) — to everyone in an "email storm" thread if it detects 10 'reply all'-s sent to over 5000 recipients within a period of 60 minutes.
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ZDNet: Microsoft adds protection against Reply-All email storms in Office 365

Microsoft, too, has suffered two such incidents already, the first in January 2019, and a second in March 2020. The Microsoft Reply-All email storms included more than 52,000 employees, who ended up clogging the company's internal communications for hours.
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