Tech History: The Email That Shut Down Microsoft

In March 1997, a Microsoft employee accidentally sent an internal test email to the wrong distribution list—a massive one. Instead of reaching a small QA group, it went to 25,000 employees.

That would’ve been bad enough… but then came the dreaded “Reply All” avalanche. Employees, confused or curious, started hitting Reply All to ask questions, tell people to stop replying, or make jokes. The problem? Every one of those replies also went to all 25,000 inboxes—multiple times.

The internal email servers were instantly overwhelmed. Bandwidth choked, inboxes froze, and the company’s internal communications system basically became unusable for hours.

This incident became so infamous inside Microsoft that they coined a term for it: “Reply-All Storm.”  And it wasn’t the last—Microsoft suffered another huge Reply-All fiasco in 2019 that trapped tens of thousands of employees in a similar email death spiral, prompting them to roll out a “Reply All Storm Protection” feature in Office 365.

The lesson? In the corporate tech world, sometimes the biggest disasters come from the smallest clicks.