Microsoft's proprietary email and calendar server Exchange – typically utilized by Outlook – has had an upper limit for emails received by "hot recipients" for quite some time. This term encompasses users who receive over 3,600 messages in their mailbox per hour. So far, this has been a soft-limit that Microsoft has not really enforced, but starting from April, this situation will change.

Outlook logo on a black and red background

Microsoft has stated that in order to optimize email flow across mailboxes and to ensure capacity across the Exchange server, 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.

Admins will also receive new insights and reports about the process in the Exchange Admin Center, allowing them to track mailboxes going over the threshold. Microsoft does not expect a significant number of mailboxes to be affected by this change.