From April 2021, Exchange Online will apply hard limits for the number of messages a mailbox can receive per hour. The limit remains the same (3,600), but now Exchange will block the mailbox receiving any more email for an hour. The new version of the Exchange Admin Center (EAC) promises to highlight problem mailboxes so that admins can ask owners why their mailboxes receive so much email.
https://office365itpros.com/2021/02/17/exchange-online-clamps-down-high-volume-mailboxes/
