Upcoming changes to mailbox receiving limits: Hot Recipients Throttling

By February 22, 2021Microsoft Exchange

To ensure optimal mail flow performance for all customers and protect the health of Exchange services, Microsoft is updating the way in which we enforce our mailbox receiving limits.

With the growth of digital transformation and remote work, having access to emails is more critical than ever. Microsoft is committed to delivering a safe and reliable email experience for all customers, which requires us to balance our service across mailboxes.

If a mailbox or tenant is allowing extremely high message volume, this can negatively impact other customers and cause email delays. Mailboxes that exceed our thresholds will consume resources from other mailboxes. As a large and global platform, we must enforce these limits to provide an equal experience for customers.

Starting in April 2021, we will progressively roll out the new enforcement, starting at a higher threshold and incrementally lowering it until we arrive at our established limit of 3600 messages per hour, per mailbox.

What are mailbox receiving limits?

These limits control the amount of messages that a user, group, or public folder can accept in a given hour. When a mailbox exceeds the threshold, any further messages to this mailbox will be throttled. The limit resets after an hour, allowing the mailbox to receive messages again. To ensure that all parties are aware, the throttled recipient will receive a notification that they are being throttled in their Inbox, and any senders to these mailboxes will receive a non-delivery report.

These receiving limits are not new. Our documentation has specified for a long time that Exchange sets a per-mailbox threshold of 3600 messages per hour, however, we have treated this as a soft limit in the past to accommodate edge cases. View more details on our established receiving limits here.

Ways to prepare

As we begin our incremental rollout, administrators should monitor current mailbox activity to identify and address mailboxes that may be affected. Note that most users are not likely to be impacted, as only a very small percentage of mailboxes are currently hitting the threshold.

Rollout begins in April, starting at a higher throttling threshold to allow customers time to adjust their mail flow strategy. Before rollout begins, administrators will have access to a new report and insight in the Exchange Admin Center for “Mailboxes exceeding receiving limits.” From here, administrators will be able to track mailboxes that are at risk of hitting the threshold, then reach out to mailbox owners to understand or mitigate the situation.

We appreciate your patience and collaboration in helping us ensure that Exchange remains a reliable platform for all.

The Exchange Transport Team

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