MC1143991 – Limiting onmicrosoft domain usage for sending emails

Product:

Bookings, Entra, Exchange, Microsoft 365 admin center

Platform:

Online, World tenant

Status:

Change type:

Feature update, Admin impact

Links:

Details:

Summary:
Microsoft will throttle outbound emails sent from default onmicrosoft.com domains starting mid-October 2025, limiting to 100 messages per organization per 24 hours. This encourages use of custom domains. Internal emails remain unaffected. Admins should switch to custom domains and update related configurations accordingly.

Details:
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.

Change Category:
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Scope:
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Release Phase:

Created:
2025-08-29

updated:
2025-08-29

Task Type

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Docu to Check

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MS How does it affect me

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MS Preperations

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MS Urgency

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summary for non-techies**

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Direct effects for Operations**

Email Throttling
Outbound emails from onmicrosoft.com domains will be limited to 100 messages per organization per 24 hours, potentially disrupting communication with external clients and partners.
   - roles: IT Admin, Marketing Team
   - references: https://techcommunity.microsoft.com/blog/exchange/limiting-onmicrosoft-domain-usage-for-sending-emails/4446167

Non-Delivery Reports (NDRs)
Once the email limit is reached, users will receive NDRs with error code 550 5.7.236, leading to confusion and frustration among users trying to send important emails.
   - roles: End Users, Support Team
   - references: https://techcommunity.microsoft.com/blog/exchange/limiting-onmicrosoft-domain-usage-for-sending-emails/4446167

Reconfiguration of Applications
Applications relying on MOERA-based usernames (e.g., Bookings) may require reconfiguration, leading to potential downtime or loss of functionality.
   - roles: IT Admin, Application Support Team
   - references: https://techcommunity.microsoft.com/blog/exchange/limiting-onmicrosoft-domain-usage-for-sending-emails/4446167

User Experience Degradation
Users may experience degraded email service and communication issues, impacting productivity and collaboration within the organization.
   - roles: End Users, Project Managers
   - references: https://techcommunity.microsoft.com/blog/exchange/limiting-onmicrosoft-domain-usage-for-sending-emails/4446167

Compliance Monitoring Challenges
Admins will need to use Message Trace to monitor MOERA usage, which may complicate compliance reporting and monitoring processes.
   - roles: Compliance Officer, IT Admin
   - references: https://techcommunity.microsoft.com/blog/exchange/limiting-onmicrosoft-domain-usage-for-sending-emails/4446167

Configutation Options**

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Opportunities**

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Potentional Risks**

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IT Security**

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explanation for non-techies**

Microsoft is planning to limit the number of emails that can be sent from their default onmicrosoft.com domains. This change will start in mid-October 2025 and aims to encourage organizations to use custom domains for their email communications. To put it simply, imagine that the onmicrosoft.com domain is like a temporary license plate on a new car. It's fine for initial use, but for long-term driving, you need a permanent plate that represents you and your vehicle. Similarly, a custom domain is like your organization's permanent email address, giving you a unique identity and better reputation in the digital world.

The new rule will restrict outbound emails from these default domains to 100 messages per organization every 24 hours. If this limit is exceeded, the sender will receive a notification that their email was not delivered. This is like a speed limit on a road; if you go over the limit, you get a warning and need to slow down. Internal emails within the organization and emails coming in from outside will not be affected by this change.

To prepare for this, organizations should identify which mailboxes are using the onmicrosoft.com domain and switch them to a custom domain. This involves purchasing a custom domain, setting it as the default in the Microsoft 365 Admin Center, and updating user mailboxes to use this new domain. Think of it as moving from a temporary office space to a permanent one, where you update your address so clients know where to find you.

Additionally, any applications or devices that rely on the onmicrosoft.com domain for authentication, like Bookings or Microsoft notifications, will need to be reconfigured to use the new custom domain. This ensures that everything continues to run smoothly, much like updating your contact information with service providers when you move to a new home.

By making these changes, organizations can maintain smooth email operations and improve their email deliverability. This proactive approach helps prevent any disruptions that might occur due to the new limitations on the onmicrosoft.com domain.