Table of Contents
Summary
- Microsoft is implementing throttling for outbound email from onmicrosoft.com addresses to enhance email reliability and reduce misuse.
- Organizations exceeding 100 outbound messages to external recipients within a 24-hour period will experience throttling.
- Use of verified custom domains for external communication is encouraged, with no changes for emails sent from these domains.
- Admins should review email usage, transition to a verified custom domain, and update users and services accordingly.
- The change impacts both users and administrators, requiring preparation to avoid service disruptions.
Primary Service: Exchange
Admin Impact: High
User Impact: Medium
Release Start: 01 Jun 2026
Release End: 15 Jun 2026
Services: Exchange
Category: Plan for change
Tags: User Adoption, Admin Action
History
5/6/2026 Item Added to Message Center
Microsoft Message
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.