Onmicrosoft.com outbound email sending limit

[Introduction]

To improve email reliability and reduce misuse of default tenant domains, Microsoft is introducing throttling for outbound email sent from onmicrosoft.com addresses. This change encourages the use of verified custom domains for external communication and aligns with best practices for domain reputation and deliverability. Following our notification in Message Center Post (MC1143991) in August last year, your organization is still sending messages using your onmicrosoft.com domain.

[When this will happen:]

We will begin rolling out in early June 2026 and expect to complete by mid-June 2026.

[How this affects your organization:]

Who is affected:

Admins managing Exchange Online and any users or services sending outbound email to external recipients using onmicrosoft.com addresses.

What will happen:

• Outbound email from onmicrosoft.com addresses will be throttled if the organization exceeds 100 messages in a 24-hour period.
• This limit applies only to messages sent to external recipients.
• There is no change for emails sent using verified custom domains.

[What you can do to prepare:]

• Review your tenant’s outbound email usage.
• Identify users, apps, or services sending from onmicrosoft.com domains.
• Set up and use a verified custom domain (for example, yourcompany.com) instead of onmicrosoft.com addresses.
• Set the custom domain as the default in the Microsoft 365 admin center.
• Update affected users and services to send from the custom domain.
• Use Message Trace in Exchange Admin Center to monitor usage and identify impacted senders.
• For detailed setup steps, refer to MC1143991 and the Exchange Team blog: Limiting Onmicrosoft Domain Usage for Sending Emails.

[Compliance considerations:]

No compliance considerations identified, review as appropriate for your organization.