Microsoft Introduces Limits on Outbound Emails from Onmicrosoft.com Domains

Microsoft Online Email Routing Address (MOERA) domains, like contoso.onmicrosoft.com, are designed to let organizations get started quickly and test a new tenant in Microsoft 365. They don’t reflect a brand’s identity and shouldn’t be used for regular email traffic. On top of that, spammers often exploit new onmicrosoft tenants to send bursts of spam, degrading the reputation of the shared domains.

To protect brand trust and improve email deliverability, Microsoft is now introducing new limits on the use of onmicrosoft.com domains for sending emails. Let’s dive into these outbound email limits in Microsoft 365, how they work, and the actions admins need to take to prepare for this update.

Until now, Microsoft has not imposed limits on the use of MOERA domains for email delivery, even though they are not suitable for regular messaging and are mainly intended for testing mail flow. However, as misuse of these domains has increased, Microsoft is now setting throttling limits for sending emails to external recipients.

Here’s what you need to know about the new throttling limits and how they apply:

• External emails sent from MOERA domains are now limited to 100 recipients per organization per 24-hour rolling window.
• No limit for internal and inbound email traffic, so messages within your organization or received from external MOERA domain senders are unaffected by the throttling.
• If a sender exceeds the limit, they will receive a Non-Delivery Report (NDR) with error code 550 5.7.236.
• When sending to a group, all external recipients in the group are counted individually. For example, if you send an email to a group with 25 external members, it counts as 25 external recipients.
• Journaling Reports in Exchange Online use a fixed Microsoft system address that admins can’t change. So, they won’t be affected by the new throttling limits.
• In hybrid setups, MOERA addresses (like mail.onmicrosoft.com) might still appear in scenarios like out-of-office replies. These cases won’t be throttled either, as long as the domain isn’t used for sending original emails.

The general availability of the new throttling begins mid-October 2025 and will be completed by early June 2026 worldwide. The timing will vary for organizations based on the number of licensed mailboxes in Exchange Online. This change will be on by default in all Microsoft 365 organizations.

For a detailed breakdown, refer to the table below.

Microsoft will announce the rollout for each stage one month in advance via Message Center to all customers meeting the licensed mailbox count criteria.

If your organization uses MOERA domains to send emails, follow these steps to prevent potential issues:

1. Purchase and configure a custom domain to ensure it represents your brand identity.
2. Use Message Trace in Exchange admin center to identify all mailboxes sending emails externally from MOERA domains. Enter the onmicrosoft.com domain (e.g., @contoso.onmicrosoft.com) in the Senders field. This will capture both internal and external messages. However, you can filter out internal emails by specifying an external domain in the Recipient field.
   
3. Configure your custom domain as the default domain in the Microsoft 365 admin center. This prevents scenarios where the Sender Rewriting Scheme (SRS) uses the MOERA domain as a fallback if it remains the default.
4. Add aliases to each mailbox with the new custom domain. Also, set the new alias as the primary SMTP address for outgoing emails.
5. Check and update authentication credentials on all devices and apps that currently use MOERA-based usernames.
6. Configure Bookings and other apps to use the custom domain for sending invites.
7. Set up the custom domain to send notifications from Microsoft 365 products.
8. For customers with Federated Domains, add a non-Federated custom domain in Microsoft 365 to act as the default domain, since Federated domains cannot serve that role.

This new throttling enforcement gives Microsoft 365 admins a smarter way to manage outbound emails and protect the organization’s brand reputation. By moving from MOERA to a custom domain, you can ensure your messages are delivered reliably and securely. Start planning your migration today to stay ahead of potential issues.

If you have any doubts, please share them in the comments. We’re happy to help! Stay tuned with us for all the latest Microsoft 365 updates!