https://www.bleepingcomputer.com/news/microsoft/microsoft-rolls-out-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.