Exchange Online already imposes limits on the number of messages a mailbox can receive per hour. New limits will restrict the number of messages individual senders can send to a third of the overall limit. The restriction doesn’t apply to senders with an Exchange Online mailbox in the same tenant. And if a mailbox runs into a limit, it features on the splendidly named Hot Recipients report. What’s not to like about that.
https://office365itpros.com/2021/07/29/new-sender-recipient-limits-exchange-online-september-2021/
