What you’re seeing
- Recipients see your messages coming from an address ending in onmicrosoft.com instead of your company domain.
- Customers report your emails land in spam or never show up at all.
- Non-delivery reports mention a sending limit or say the message was throttled, often after a batch of outgoing mail.
- Automated mail from a shared mailbox, scanner or app tied to the default tenant domain stops going out.
Common causes
Every environment is different, but these are the causes our engineers find most often:
- Users or shared mailboxes still have the onmicrosoft.com address as their primary sending address because the custom domain was never made the default.
- Microsoft caps outbound volume from the shared onmicrosoft.com namespace to protect its reputation, and your tenant has reached that cap.
- Receiving systems distrust onmicrosoft.com mail because spammers frequently abuse newly created trial tenants.
- Your custom domain was added to Microsoft 365 but never fully verified, so it isn't available as a sending address.
- An application, copier or website form was configured to send through the default tenant address rather than a mailbox on your own domain.
Why it matters
Mail from onmicrosoft.com looks unprofessional at best and suspicious at worst. Customers may ignore it or never see it, and once Microsoft throttles the tenant, quotes, order confirmations and scanned documents stop leaving entirely. Because the address is shared across Microsoft's customers, its reputation is outside your control, so the problem can come and go unpredictably. Sending from your own domain is also what lets you protect your brand with SPF, DKIM and DMARC.
How AskTechGurus fixes it
Skip the trial and error. A certified engineer works on your actual system, finds the root cause, and fixes it properly.
Tell us what’s happening
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A certified engineer diagnoses it
A vetted, US-based systems engineer claims your ticket, usually within minutes during business hours, and investigates over a secure remote session you can watch and end at any time.
Fixed and verified with you
The engineer resolves the root cause and confirms with you that it’s working. You don’t just get a workaround, and you get a plain-English summary of what was wrong.
Pay only when it’s resolved
Fixed price from $99, invoiced only after you approve the fix. If it isn’t resolved, you don’t pay.
Frequently asked questions
Why does Microsoft limit mail from our onmicrosoft.com address?
That domain is shared by every Microsoft 365 tenant and is a frequent target for abuse, so Microsoft restricts how much mail it can send. It's meant for setup and testing, not daily business email.
Will switching to our own domain change how people sign in?
It doesn't have to. Email addresses and sign-in names can be handled separately, and we plan the change so users keep signing in the way they're used to.
Will mail sent to the old address still arrive?
In a typical setup, yes. The onmicrosoft.com address can remain on the mailbox as a secondary address so nothing sent to it is lost.