MC272450 – Sender-recipient pair receiving limits in Microsoft Office 365

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check before: 2021-08-06

Product:

Exchange, Microsoft 365 admin center

Platform:

Online, World tenant

Status:

Change type:

Admin impact, User impact

Links:

MC239262

Details:

We are updating our receiving limits in Exchange to help prevent attacks on your mail flow experience. Earlier this year in (February MC239262) we announced a stricter enforcement of our mailbox receiving limits. Taking your feedback into consideration, we are releasing an additional limit to block single-sender mail storms and deter DoS attacks.
Our mailbox receiving limits, as previously stated, apply to the messages received by a Microsoft Office 365 mailbox. If volume exceeds 3,600 messages in a given 60-minute window, the mailbox will no longer accept messages from the Internet, from other tenants, or from on-premises senders.
Starting in September 2021, we are adding a limit on sender-recipient pairs (SRP). This feature will apply to the messages received by a Microsoft Office 365 mailbox from each specific sender. If a single sender sends over 33% of the threshold (3,600 per rolling hour) to a specific recipient, the SRP limit will kick in, and the mailbox will no longer accept messages from that sender. The mailbox will continue accepting messages from other senders.
Note: If the identified sender is from a Microsoft Office 365 mailbox in the same tenant, messages will be allowed even after the limit is exceeded. If the identified sender is from an on-premises mailbox, a Microsoft Office 365 in a separate tenant, or outside of Microsoft Office 365, messages will be blocked.
This change helps prevent a malicious user from blocking mail flow to a Microsoft Office 365 mailbox, as part of our continuing efforts to improve your Exchange Online experience.
[Key Points:]
Timing: September 2021
Action: review and assess

Change Category:
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Scope:
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Ring:

Created:
2021-07-23

updated:
2021-07-23

task type:
Plan For Change

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