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M365 Changelog: Enforcing mailbox receiving limits

Petri Staff


MC239262 – Updated April 29, 2021: Microsoft has updated the rollout timeline below. Thank you for your patience.

Microsoft is updating the way they enforce their receiving limits in Exchange. Starting in May 2021, Microsoft will be more strictly enforcing limits across all mailboxes to protect the health of their 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 Microsoft’s published and established threshold of 3600 messages per hour. While this is not a new limit, it has been a soft limit before – Microsoft has ensured up to 3600 but in some cases supported higher volume. Microsoft is now making this a hard limit.

Key Points:

  • Timing: May 2021
  • Action: review and assess

How this will affect your organization:

Rollout will begin for all tenants at the same time, and Microsoft will enforce limits incrementally by starting with a higher throttling threshold, then gradually lowering the threshold until Microsoft reaches the established limit. This gives you time to reach out to users that are exceeding the limit, which you’ll be able to monitor in the Exchange Admin Center through a new “Mailboxes exceeding receiving limits” insight and report.

Note: Most users are not likely to be impacted by this, as only a small percentage of mailboxes are currently hitting receiving limits.

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. The insight and report will be released end of March, prior to when rollout of limit enforcement begins.

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 via the “Insights” tab.

From this insight, or from the “Reports>Mail flow” tab, 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.

What you need to do to prepare:

No direct action is required on your part, though it is recommended that you review the established limits and update training and documentation as appropriate.

  • Review the published receiving limits at Exchange Online limits – Service Descriptions | Microsoft Docs
  • Inform users of actions they can take to reduce mail load
  • High mail load is often caused by automated mailers. If an automated mailer is sending high volume to a single mailbox, users should configure the mailer to deliver across a longer period or to multiple mailboxes.

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