Microsoft to Restrict Use of OnMicrosoft Domains for Email Sending

Microsoft has announced significant restrictions on the use of default onmicrosoft.com domains for email communication, implementing new throttling measures to combat spam and improve email deliverability across its Microsoft 365 platform.

Policy Changes Target Spam Prevention

The technology giant will introduce throttling limits that restrict messages sent from onmicrosoft.com domains to just 100 external recipients per organization within a 24-hour rolling window.

This represents a major shift from Microsoft’s previous approach, which imposed no limits on Microsoft Online Email Routing Address (MOERA) domain usage for email delivery.

The change addresses a persistent problem where spammers exploit newly created tenants to send large volumes of spam from onmicrosoft.com addresses before Microsoft can intervene. 

This abuse degrades the shared domain’s reputation, negatively impacting all legitimate users who rely on these default domains for business communications.

When organizations exceed the new 100-recipient limit, they will receive Non-Delivery Reports (NDRs) with error code 550 5.7.236 for any attempts to send additional external emails while throttled.

Internal messaging and inbound communications will remain unaffected by these restrictions.

Organizations must take immediate action to avoid disruption to their email operations. Microsoft recommends that customers purchase and migrate to custom domains, ensuring that only branded domains are used for non-test email communications.

Companies currently using MOERA domains as their primary domain must establish custom alternatives and update their primary SMTP addresses accordingly.

The restrictions will impact various Microsoft 365 features beyond standard email sending. 

Sender Rewriting Scheme configurations, Bookings app invitations, and Microsoft product notifications may all be affected if they rely on MOERA domains.

Organizations using federated domains face additional complexity, as they must add non-federated custom domains to serve as default domains.

Microsoft has identified several scenarios where MOERA domains remain necessary, including journaling reports and hybrid configurations with complex routing. These specific use cases will not count toward the throttling limits.

The implementation follows a staggered rollout schedule based on Exchange seat counts.

Trial organizations face restrictions starting October 15, 2025, while larger enterprises with over 10,000 seats have until June 1, 2026, to comply. Smaller organizations with fewer than three seats will see throttling begin December 1, 2025.

Microsoft will provide one-month advance notice through Message Center announcements before each rollout phase begins. 

The company strongly encourages all customers currently using MOERA domains to begin planning their migration immediately to avoid potential email delivery disruptions.

Organizations can analyze their current MOERA email traffic using the Message Trace feature in Exchange Admin Center to assess their exposure and plan accordingly for the transition to custom domains.

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