Microsoft is planning to add protection against Reply-All email storms to Office 365. The new feature will automatically block users from replying to each other for a limited amount of time. Microsoft employees recently fell victim to reply-allpocalypse during January 2019, with over 11,500 of them being caught up in a giant email thread. A possible outcome of such an event is an inadvertent DDoS attack that can potentially take down one or more email servers used to deliver the huge amounts of replies.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-office-365-to-add-reply-all-mail-storm-protection/