Microsoft to throttle Exchange mailboxes


Staff member
Jan 8, 2011
Microsoft has stated that in order to optimize email flow across mailboxes and to ensure capacity across the Exchange services, it will begin enforcing its existing limit of being able to receive 3,600 messages per hour. The company says that when tenants and mailboxes go beyond this limit, services for other customers are disrupted as well, causing delays in emails being received due to network resources being utilized by "hot recipients".

The Redmond tech giant says that once it begins to throttle tenants, emails sent to full mailboxes will receive a non-delivery report. The threshold will be automatically reset every hour. According to the dedicated webpage, this limit applies to all of the following subscriptions:
  • Microsoft 365 Business Basic
  • Microsoft 365 Business Standard Office
  • Office 365 Enterprise E1
  • Office 365 Enterprise E3
  • Office 365 Enterprise E5
  • Office 365 Enterprise F3
As Microsoft begins to enforce this limit starting in April 2021, it has encouraged admins to keep an eye on activity across mailboxes. The firm will start with a higher threshold and keep lowering it incrementally until it reaches the official limit of 3,600 messages per hour so organizations have time to adapt to the change.

Microsoft Exchange Team Blog