Published June 30, 2021

Microsoft Defender for Office 365: Extending Secure by Default for Exchange Transport Rules

Reference ID: MC265759 | Effective: August 1, 2021

MC265759 – Microsoft believes it’s critical to keep their customers secure by default. Microsoft has determined that legacy overrides tend to be too broad and cause more harm than good. As a security service, Microsoft believes it’s imperative that they act on your behalf to prevent your users from being compromised. This means these legacy overrides will no longer be honored for email messages Microsoft believes are malicious. They already apply this approach with malware messages and now have extended it to messages with high confidence phish verdicts. Microsoft have been taking a very deliberate approach to rolling out these changes in phases to ensure customers are not surprised and there are no negative side effects. Microsoft began to rollout Secure by Default for high confidence phishing messages by the override type starting in December 2020 (Roadmap ID 60827). Today, Microsoft is at a point in their Secure by Default journey where the following overrides are not honored for malicious emails (malware or high confidence phish emails):

  •  Allowed sender lists or allowed domain lists (anti-spam policies)
  •  Outlook Safe Senders
  •  IP Allow List (connection filtering)

Microsoft is now extending Secure by Default to cover high confidence phishing messages for the remaining legacy override type, Exchange mail flow rules (also known as transport rule or ETRs). 

Key Points

  • Timing: Microsoft will begin rolling out Secure by Default for ETRs starting at the beginning of August and complete rollout by early September.
  • Action: Review and assess impact. 

How this will affect your organization:

After the last phase of Secure by Default is enabled in August for ETRs, Defender for Office 365:

  • Will no longer deliver messages with a high confidence phish verdict, regardless of any explicit ETRs.
  • Will no longer recommend using ETRs to configure third-party phishing simulations or Security Operations mailbox message delivery.

What you need to do to prepare:

If you are currently using Exchange mail flow rules (also known as transport rules or ETRs) to configure your third-party phishing simulation campaigns or delivery for security operation mailboxes, you should begin to configure these with the new Advanced Delivery policy when the feature is launched in July (Roadmap ID 72207). For more information, please refer to message center post MC256473. Administrators should also use the submission portal to report messages whenever they believe a message has the wrong verdict so that the filter can improve organically.

Learn more:

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