If you’ve still got mailboxes or services firing off emails from something@yourtenant.onmicrosoft.com, consider this your polite nudge (well, Microsoft’s) to stop.
Because in classic Microsoft fashion, it’s not just a suggestion anymore — they’re throttling it.
Yes, seriously!
The Change
Starting October 15, 2025, Microsoft will start throttling outbound email sent from .onmicrosoft.com addresses to 100 recipients per tenant, per day. It’s a phased rollout, with full enforcement by June 2026.
After that? Every message over the limit gets bounced faster than your expense claim for a “technical lunch” at Gaucho.
🧾 Full details here on TechCommunity
Why the Sudden Crackdown?
To be fair, Microsoft’s letting you down gently. The reasons behind this move are solid:
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Shared reputation – Your
.onmicrosoft.comdomain shares an IP rep with every other tenant. That includes legitimate businesses… and also dodgy spam farms. -
Trust and branding – No one feels good getting an invoice from
accounts@widgets-inc.onmicrosoft.com. It just doesn’t inspire confidence. -
Security – Spoofing an
onmicrosoft.comaddress is relatively easy for attackers. This change makes that harder — and forces orgs to clean up their setup.
What Actually Breaks?
Here’s the fun bit: it’s per tenant, not per user.
So if multiple users or automated services are still sending from @onmicrosoft.com, you’ll all be queuing for that same 100-email daily allowance. Go over, and Microsoft slaps you with this lovely NDR:
550 5.7.236– Message rejected due to sending limits.
That means:
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Support mailboxes stop replying
-
CRM notifications don’t arrive
-
Your legacy scanner in Accounts can’t send its daily scan of someone’s elbow
What You Should Be Doing Instead
This really shouldn’t be news. But hey, if your setup still leans on the freebie domain, here’s your to-do list:
✅ Register a Real Domain
Use something official — ideally the same domain your users sign into.
No myrealbusinesssolutions365v2.biz, please.
✅ Add It to Microsoft 365
Go to Admin Centre > Settings > Domains and follow the prompts.
Set up your DNS records — SPF, DKIM, DMARC — all the good stuff.
✅ Set As Default
Make sure new users and services get assigned your real domain automatically — not @onmicrosoft.com.
✅ Fix Existing Mailboxes
Use PowerShell to change addresses:
Don’t forget to double-check login UPNs and app dependencies.
One careless change and suddenly half your staff can’t log into Teams.
✅ Audit Everything Sending Mail
Check for services, apps, Power Automate flows, old scanners, or hybrid mail relays still sending from the wrong domain. Microsoft’s Message Trace or Defender XDR can help.
But… Why Was I Using It Anyway?
Short answer: because it was easy.
Long answer: it was easy 10 years ago.
The .onmicrosoft.com domain was always meant to be a placeholder — for testing, tenant setup, and temporary use. Not for external mail, marketing comms, or service account spam.
Would you send corporate mail from yourbusiness@hotmail.com?
(…don’t answer that if you’re still doing it.)
Bonus Round: Do Some Security While You’re There
While you’re cleaning up your domain usage, it’s a great time to:
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✅ Set up SPF to say who can send on your behalf
-
✅ Enable DKIM to sign your mail
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✅ Configure DMARC so spoofers get blocked
-
✅ Add a Transport Rule to stop future sends from
.onmicrosoft.comjust in case someone tries again
You’ll sleep better at night — promise.
Final Thought
If you haven’t sorted this already, don’t worry — there’s still time. But make no mistake, this change is coming whether you’re ready or not. And while fixing it might feel like a chore, not fixing it is worse.
Avoid outages, broken processes, and embarrassing email bounces.
Use a real domain. Email like a grown-up.
Your support desk will thank you. And so will your customers.
TL;DR
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Microsoft is throttling
.onmicrosoft.comemail sends from October 2025 -
The cap is 100 recipients per tenant per day
-
Use your real domain — now
-
Audit your setup and fix anything that sends from the default tenant domain
-
Update SPF/DKIM/DMARC while you’re there