Microsoft Announces Next Level of Service Protection Throttling

The incident MC239262 in the Office 365 Admin Portal gives insight on upcoming changes to Service Protection Throttling

Enforcing mailbox receiving limits

MC239262 | February 13 - We are updating the way we enforce our receiving limits in Exchange. Starting in April 2021, we will be more strictly enforcing limits across all mailboxes to protect the health of our systems and ensure optimal mail flow performance for all customers.Mailboxes that receive high message volume in a short time can cause mail flow delays, both for individual recipients and across your organization.With these changes, emails will be throttled for recipients that exceed our published and...

With these changes, emails will be throttled for recipients that exceed our published and established threshold of 3600 messages per hour. While this is not a new limit, it has been a soft limit before – we’ve ensured up to 3600 but in some cases supported higher volume. We are now making this a hard limit.

If a mailbox exceeds the established limit, messages to that mailbox will be throttled. Affected mailboxes will receive an email informing them of the throttling, while senders to these mailboxes will receive a non-delivery report specifying that the recipient has been getting too many messages. Emails will be throttled until the limit resets one hour from when the threshold was exceeded, allowing the mailbox to accept messages again.

Administrators will be able to monitor users that exceed their receiving limits through the new “Mailbox exceeding receiving limits” insight and report in the Exchange Admin Center. When one or more mailboxes in your organization hit the receiving limit, an insight titled “Some mailboxes need attention: one or more mailboxes have exceeded their receiving limits” will appear in your dashboard.

From this insight you can open the “Mailbox exceeding receiving limits” report, showing mailboxes that have hit or are at risk of hitting the receiving limit in the last 24 hours. You will also find the mailbox owner’s contact information. Please contact affected users to understand why they are receiving so many messages and inform them of ways to reduce mail volume and improve their experience.

3600 items per hour is just 1 item per second. This new limit is enforced now on top of the 150 Mb/min announced previously for service protection throttling. While it won't matter for normal usage of O365, it will matter for onboarding and migration projects to M365.

This is the latest example of Microsoft introducing changes that help protect the service it provides with Office 365 - they call it service protection throttling. We've talked about this before in a previous blog post which describes a change that Microsoft implemented recently. Cloudficient adapted to that change rapidly, and we are already working hard to test and address any required changes that arise due to this recent announcement from Microsoft.

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We have an in-depth review of service protection throttling in another article. Click here to read that.

Update: 22nd February 2021 - You can also read about it on the Microsoft Exchange blog and they term it Hot Recipients Throttling.