check before: 2021-08-06
Exchange, Microsoft 365 admin center
Online, World tenant
Admin impact, User impact
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.
Timing: September 2021
Action: review and assess
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Last updated 1 month ago