What is the onmicrosoft.com domain in Microsoft 365?
In the Microsoft 365 ecosystem, every new tenant automatically receives an onmicrosoft.com domain, known as a MOERA (Managed Onmicrosoft Email Routing Address).
This domain enables quick activation of the environment and the initial creation of users, but it is not intended for regular email sending.
Why does Microsoft limit the use of the onmicrosoft.com domain?
Microsoft has identified that MOERA domains are frequently used by malicious actors to send spam from newly created tenants before they can be shut down.
This has led to a degradation of the shared onmicrosoft.com domain’s reputation, affecting even legitimate organizations that were using it as their primary domain.
Technical restrictions applied by Microsoft
- Emails sent from onmicrosoft.com domains are limited to 100 external recipients per organization within 24 hours.
- Incoming messages are not affected.
- When the limit is exceeded, the sender receives an NDR with error code 550 5.7.236.
Microsoft 365 recommendations
Microsoft recommends that all organizations:
- Acquire and configure a custom domain that represents their brand.
- Set the custom domain as the default in the Microsoft 365 Admin Center.
- Update the primary SMTP addresses of mailboxes to reflect the new domain.
- Review authentication credentials on devices and apps that depend on the previous alias.
This change not only improves email deliverability but also strengthens corporate identity and administrative control.
Microsoft’s official position on the onmicrosoft.com domain
Microsoft’s official documentation reinforces this position with several recommendations:
- The onmicrosoft.com domain should be considered a technical fallback, useful for testing or contingencies, but not for production.
- A verified custom domain is recommended for sending notifications and internal communications, as it improves reputation and prevents emails from being flagged as spam.
- In hybrid environments, mail flows and subdomains can be managed through advanced configurations in Exchange Online.
Migrating to a custom domain is a necessity
The limitation on the use of the onmicrosoft.com domain is not an arbitrary restriction, but a strategic measure to protect the reputation of the Microsoft 365 ecosystem and ensure the security and effectiveness of business communications.
Adopting a custom domain not only strengthens your brand image but also protects your organization from risks and compliance issues.
ABD Consulting: your partner for managing domains in Microsoft 365
At ABD IT Consulting and Solutions we help companies:
- Migrate from onmicrosoft.com to a secure and reliable custom domain.
- Configure Microsoft 365 professionally to optimize corporate email management.
- Provide guidance on data protection and digital security aligned with Microsoft best practices.
Ready for your organization to move beyond the limitations of onmicrosoft.com? Get in touch with ABD and take the step toward a more secure, efficient, and professional email management experience.