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. The Reply All Storm Protection feature will send the bounce message to everyone in an “NDR”” thread if it detects 10’reply all’-s sent to over 5000 recipients within a period of 60 minutes. Users will be told that their reply was not delivered and that they are required to stop using the Reply All feature and send it to smaller number of recipients instead of the entire email list.”
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-rolls-out-protection-against-office-365-email-storms/