What’s Changing?
Microsoft has officially announced via their Exchange Team Blog that default onmicrosoft.com domains (MOERA domains) will now be limited to 100 external recipients per organization per 24-hour rolling window. (Microsoft Exchange Team Blog)
When the limit is exceeded, senders will receive a Non-Delivery Report (NDR) with the error code 550 5.7.236. Internal communications remain unaffected.
Why Is Microsoft Doing This?
The primary motivation is to protect the shared reputation of the onmicrosoft.com domain. Spammers often exploit newly created tenants to send high volumes of junk mail, and when those are flagged, the entire namespace suffers. By throttling, Microsoft aims to:
- Improve Deliverability: Prevent legitimate emails from being blocked due to shared reputation damage.
- Combat Spam: Limit the effectiveness of spammers abusing trial tenants.
- Encourage Best Practices: Drive organizations to adopt a custom domain, which is essential for brand identity, credibility, and secure email communication.
Rollout Timeline
Enforcement will be phased by tenant size:
| Phase | Rollout Date | Tenant Size (Exchange Seats) |
|---|---|---|
| 1 | Oct 15, 2025 | Trial tenants |
| 2 | Dec 1, 2025 | Fewer than 3 seats |
| 3 | Jan 7, 2026 | 3–10 seats |
| 4 | Feb 2, 2026 | 11–50 seats |
| 5 | Mar 2, 2026 | 51–200 seats |
| 6 | Apr 1, 2026 | 201–2,000 seats |
| 7 | May 4, 2026 | 2,001–10,000 seats |
| 8 | Jun 1, 2026 | 10,000+ seats |
Administrators will be notified via the Microsoft 365 Message Center at least 30 days before enforcement begins.
Actionable Steps for Administrators
1. Get a Custom Domain
- Purchase a domain (e.g., yourcompany.com).
- Add and verify it in the Microsoft 365 admin center.
- Configure DNS records (MX, SPF, DKIM, CNAME) as instructed.
2. Update Mailboxes and Accounts
- Change primary SMTP addresses from
user@tenant.onmicrosoft.comtouser@yourcompany.com. - Update shared mailboxes, groups, and distribution lists.
3. Audit Integrations and Services
- Update Microsoft services (like Bookings) that default to MOERA.
- Review third-party apps and scripts that rely on onmicrosoft.com.
4. Communicate and Educate
- Notify employees of the new domain.
- Update email signatures, templates, and branding.
FAQ: Your Questions Answered
Q: My organization is small. Do we really need a custom domain?
A: Yes. This policy makes clear that using onmicrosoft.com for professional communication is not sustainable. Even the smallest business can register and use a custom domain at minimal cost. It is essential that you do so.
Q: Will this affect internal emails?
A: No. The throttling applies only to emails sent to external recipients.
Q: What about a trial tenant needing to send more than 100 emails?
A: Trials are for evaluation only. For higher volumes, configure a custom domain.
Q: What if we don’t adopt a custom domain by the deadline?
A: External email sending will be severely restricted, causing business disruption.
Why do this?
- Deliverability: Protects inbox placement and brand reputation.
- Spam Prevention: Raises the cost of abuse for spammers.
