💡 Our Technical Review in summary
- Microsoft is introducing outbound email throttling for default Microsoft Online Email Routing Address (MOERA) domains (e.g., tenant.onmicrosoft.com) to improve global email deliverability and reduce service abuse.
- These domains are intended strictly for initial setup and testing, but many organizations are currently using them for production traffic, which risks the reputation of the shared domain space.
- A strict limit of 100 outbound messages to external recipients per organization per 24-hour rolling window will be enforced starting in late 2025.
Impact
- Throttling Limits: Once an organization exceeds the 100-message limit to external recipients, any further outbound emails from the onmicrosoft.com domain will be blocked.
- Error Codes: Blocked senders will receive Non-Delivery Reports (NDRs) with error code 550 5.7.236.
- Scope: This change only affects outbound mail to external recipients. Internal email traffic (within the same tenant) and inbound email traffic will remain unaffected.
- Timeline: Rollout begins mid-October 2025 and is expected to be complete by early June 2026. The specific timing depends on the organization’s seat count.
- Application Disruptions: Automated systems, Microsoft Bookings, and third-party applications or devices (like scanners) configured to send via MOERA addresses may experience delivery failures.
Action Required
- Audit Usage: Use the Message Trace tool in the Exchange Admin Center (EAC) to identify all mailboxes, applications, and automated processes currently sending external mail via the onmicrosoft.com domain.
- Custom Domain Implementation: If not already present, purchase and verify a custom domain (e.g., @contoso.com) within the Microsoft 365 Admin Center.
- Update SMTP Addresses: Change the primary SMTP address for all users from the MOERA domain to the verified custom domain.
- Reconfigure Applications: Update the “Send As” or “From” settings in Microsoft Bookings, notification services, and local devices (SMTP relay/MFP) to use the custom domain.
- Set Default Domain: Ensure the custom domain is set as the default domain for the tenant to prevent new users from being automatically assigned a MOERA address as their primary identity.
Microsoft Official Update
Service: N/A
Category: preventOrFixIssue
Severity: normal
Introduction
To improve email deliverability and reduce abuse, Microsoft is introducing outbound email throttling for default MOERA domains (e.g., contoso.onmicrosoft.com). These domains are intended for initial tenant setup and testing—not for regular email traffic. This change encourages organizations to adopt custom domains with independent reputations.
When this will happen
General Availability (Worldwide): Rollout begins mid-October 2025 and will complete by early June 2026. Rollout timing will vary based on organization seat counts. For detailed rollout information, refer to Limiting Onmicrosoft Domain Usage for Sending Emails.
How this affects your organization
Organizations sending outbound email from onmicrosoft.com domains will be subject to throttling limits:
- Outbound email to external recipients will be capped at 100 messages per organization per 24-hour rolling window.
- Once the limit is reached, senders will receive non-delivery reports (NDRs) with error code 550 5.7.236.
- Internal and inbound email traffic will not be affected.
This change is on by default and does not require admin enablement.
What you can do to prepare
Take the following actions:
- Use Message Trace in the Exchange Admin Center to identify mailboxes using MOERA domains.
- Purchase and configure a custom domain for your organization.
- Set the custom domain as the default domain in the Microsoft 365 Admin Center.
- Update user mailboxes to use the custom domain as their primary SMTP address.
- Review and update authentication credentials for devices and apps using MOERA-based usernames.
- Ensure apps like Bookings and Microsoft notifications are configured to use your custom domain.
Learn more: Limiting Onmicrosoft Domain Usage for Sending Emails
Compliance considerations
| Compliance Area | Explanation |
|---|---|
| Does the change modify, interrupt, or disable any of the following capabilities (Purview)? | Applications relying on MOERA-based usernames (e.g., Bookings, Microsoft notifications) may require reconfiguration to maintain functionality. |
| Does the change alter how admins can monitor, report on, or demonstrate compliance activities? | Admins must use Message Trace in the Exchange Admin Center to identify MOERA usage and monitor throttling impact. |
| Does the change include an admin control and, can it be controlled through Entra ID group membership? | Admins can configure custom domains and update SMTP addresses; however, Entra ID group control is not explicitly mentioned. |
