Why You Need to Stop Using onmicrosoft.com for Emails in Microsoft 365

When you spin up a new Microsoft 365 tenant, Microsoft automatically gives you a default domain — usually something like thatlazyadmin.onmicrosoft.com. These MOERA (Microsoft Online Email Routing Address) domains are incredibly handy at the beginning. They let you test, create users, and send mail right away, even before you’ve bought a proper domain.

But here’s the problem: too many organizations never move away from that default. They keep using onmicrosoft.com addresses for daily business communication, either out of convenience or because no one stopped to review the configuration after setup.

Microsoft has now made it clear that this practice is coming to an end. And it’s not just about brand reputation — it’s about security, deliverability, and making sure your tenant doesn’t get caught up in the wrong side of Microsoft’s anti-abuse measures.

Why This Change Matters

Let’s be blunt: sending from onmicrosoft.com was never a good idea.

• It doesn’t represent your brand. Customers see thatlazyadmin.onmicrosoft.com and immediately know it’s a default tenant setup, not a professional domain.
• It hurts deliverability. Because all tenants share the same onmicrosoft.com namespace, if spammers abuse the domain (and they do), your emails can be treated with suspicion too.
• It limits your control. You can’t properly apply SPF, DKIM, and DMARC on these addresses, which means you’re missing critical layers of email authentication and security.
• Microsoft is now enforcing limits. Starting October 2025, outbound mail from onmicrosoft.com will be throttled to just 100 external recipients per day per tenant. That’s a hard stop for any organization still relying on these addresses.

What Microsoft Is Changing

Microsoft is introducing outbound throttling for MOERA domains. Here’s what you need to know:

• Throttle limit: 100 external recipients per tenant in a 24-hour window.
• Internal traffic: Not affected.
• Inbound mail: Not affected.
• Error messages: If the limit is hit, senders get NDRs with code 550 5.7.236.

The rollout is staggered by tenant size, starting October 15, 2025 for trials and completing by June 1, 2026 for the largest tenants.

Real-World Demo Scenario

Let’s imagine a small business — ThatlazyAdmin Consulting — that never registered a custom domain and still uses thatlazyadmin.onmicrosoft.com for all staff email.

Day 1: Sending Emails Works Fine

• A user sends out a newsletter to 85 customers.
• The mail is delivered without issue.

Day 2: Sending Limit Reached

• Later in the day, the same user tries to send an additional update to 50 suppliers.
• The mail flow hits the 100 external recipients limit.
• The sender receives a Non-Delivery Report (NDR) with:
• 550 5.7.236 – MOERA throttling limit exceeded

Business Impact

• No external emails can be sent until the 24-hour window resets.
• Internal mail still works, but customers and suppliers no longer receive communications.
• The company is essentially cut off from email as a business tool until throttling clears.

What You Should Do Now

If your organization is still sending from onmicrosoft.com, here are the practical steps to take:

1. Buy and register a proper domain
   • Use a registrar, purchase a domain that matches your business, and add it in the Microsoft 365 admin center.
2. Set the custom domain as your default
   • Make the custom domain the primary in your tenant so new users automatically get the correct address.
3. Update users’ email addresses
   • Add the new domain as an alias and then promote it to the Primary SMTP Address.
   • Watch out: this may impact login UPNs for some users, so plan carefully.
4. Reconfigure services and apps
   • Microsoft Bookings, notifications, and SRS (Sender Rewriting Scheme) may still default to onmicrosoft.com. Update them to use your custom domain.
   • Hybrid and routing setups that reference mail.onmicrosoft.com should also be reviewed.
5. Audit your usage today
   • Run a Message Trace in Exchange Admin Center with *@*.onmicrosoft.com as the sender.
   • Filter out internal mail and review what’s still flowing through MOERA. This will tell you how big of a clean-up you’re facing.

The change is coming whether you’re ready or not. Microsoft has given a clear timeline and the enforcement is hard-coded. If you’re still running on onmicrosoft.com, now is the time to clean it up, migrate your users, and make sure all your services use a proper, branded domain.

Do it before throttling starts — not after.

For the full Microsoft announcement, see:
Limiting OnMicrosoft Domain Usage for Sending Emails – Exchange Team Blog