MC1463510 – M365 Change Digest

One-line summary

Microsoft is introducing outbound external messaging limits in Teams for tenants using only the default onmicrosoft.com domain to reduce spam and abuse, effective mid-September 2026.

Details

Summary

Microsoft Teams will impose outbound external messaging limits starting mid-September 2026 for organizations using only the default onmicrosoft.com domain to reduce spam and abuse. Limits may temporarily block external messages but won't affect internal communication. Admins should consider adding a custom domain and prepare support staff.

Body (from Message Center)

[What and Why:]

To help protect Microsoft Teams users from spam, phishing, and other abusive activity, Microsoft is introducing outbound external messaging limits for organizations that use only the default Microsoft Online Email Routing Address (MOERA) domain, such as contoso.onmicrosoft.com.

These limits are designed to reduce large-scale abuse while preserving normal collaboration scenarios. Most organizations using Teams for standard business communications are not expected to reach these limits.

[Rollout Schedule:]

General Availability (Worldwide): Rollout begins in mid-September 2026 and is expected to complete by mid-September 2026.

[Impact on Your Organization:]

Who is affected

Organizations that use only the default onmicrosoft.com domain and have not configured a custom domain.

Platforms/Services

Microsoft Teams
External chat and messaging

What will happen

Outbound Teams messages sent to external users will be subject to messaging limits intended to reduce abusive or high-volume activity.
When an applicable limit is reached, users may be temporarily unable to send messages to external users.
Users will see an in-product notification indicating that outbound external messaging has been temporarily limited.
Internal Teams messaging and collaboration within your organization will not be affected.
External messaging capabilities will automatically resume when activity falls below the applicable threshold.

The protection is enabled by default and does not require administrator configuration.

An optional administrator alert for throttling events will become available after the initial rollout. This post will be updated when information about enabling the alert is available.

[Action Required / Recommendations:]

If your organization relies on the default onmicrosoft.com domain for production use, review your domain configuration before rollout.

Verify whether your organization uses only the default onmicrosoft.com domain.
If appropriate, add and configure a custom domain for your Microsoft 365 organization.
Inform help desk personnel and Teams administrators that users may encounter temporary outbound external messaging restrictions if applicable limits are reached.
Review the Teams limits and specifications documentation for additional guidance.

No action is required for organizations that already have a custom domain configured.

Additional Information

This change helps reduce spam and other abusive activity originating from tenants that use only the default onmicrosoft.com domain while maintaining normal collaboration experiences for legitimate users.

Raw JSON (for debugging)

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      "Summary": "Microsoft Teams will impose outbound external messaging limits starting mid-September 2026 for organizations using only the default onmicrosoft.com domain to reduce spam and abuse. Limits may temporarily block external messages but won\u0027t affect internal communication. Admins should consider adding a custom domain and prepare support staff."
    },
    "id": "MC1463510",
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    "last_modified": "2026-08-28T21:41:08Z",
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