Key points at a glance
- Gmail and Outlook check with every email whether the sender can identify itself.
- SPF, DKIM and DMARC are mandatory for bulk senders; everyone else is filtered more strictly without these records.
- The non-delivery notice often states the reason – it’s worth reading it carefully.
First: did you get a non-delivery notice back?
If an email is rejected, you usually get a notice back, often in English (“Undeliverable”, “Delivery Status Notification”). It contains a code, such as 550, and often a short reason. If no notice comes back, the email probably landed in the recipient’s spam folder.
- References to SPF, DKIM, DMARC or “not authenticated”: your domain isn’t identifying itself correctly.
- References to “blocked”, “listed” or the name of a blocklist: your mail server or domain is on a blacklist.
- Code 550 5.7.515 for Outlook or Hotmail addresses: Microsoft rejected the email for missing authentication.
- References to “mailbox full” or “user unknown”: the problem is on the recipient’s end, not yours.
Google’s rules (Gmail)
Since February 2024, minimum rules have applied to everyone sending to private Gmail addresses:
- Every sender needs SPF or DKIM for its domain.
- The sending server needs a valid name in the DNS (including reverse) and an encrypted connection (TLS).
- Spam complaints must stay below 0.3 percent; Google recommends under 0.1 percent.
- Anyone sending more than 5,000 emails a day to Gmail needs SPF and DKIM, plus DMARC (at least p=none) matching the sender domain, and a one-click unsubscribe for marketing emails.
Stricter since late 2025
At first, emails that didn’t meet the rules mostly landed in the spam folder. Since November 2025, Gmail delays or rejects such emails.
Microsoft’s rules (Outlook, Hotmail)
Since 5 May 2025, Microsoft has required senders who send more than 5,000 emails a day to private Outlook, Hotmail or Live addresses to have SPF, DKIM and DMARC (at least p=none, matching the sender domain). Microsoft rejects emails that don’t meet this with the code 550 5.7.515.
This threshold doesn’t apply in the same way to business mailboxes in Microsoft 365. Even there, though, the spam filter checks SPF, DKIM and DMARC and rates senders without these records more strictly.
- Sender identified?SPF, DKIM and DMARC match the sender domain.
- Server in order?Valid name in the DNS, encrypted connection, no blocklist.
- Few complaints?Hardly any recipients mark your emails as spam.
- DeliveryInbox instead of spam folder or rejection.
Free domain check
Is your company address protected?
Enter your domain or your email address. You see the result right here, with a traffic light and a grade from A to F.
You opened a link to a result. The check only starts when you click, always with the current state.
Privacy: if you enter an email address, we transmit only the domain; the part before the @ never leaves your browser. We use the domain only for this check of public DNS records and do not store it in plain text; we do not store the result. We store your IP address only as a checksum and for at most one day to curb abuse.
The quick check is based on public DNS data at the time of the query and does not replace a full review; no warranty.
Keep an eye on it continuously instead of checking once:
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- Ends on its own after 14 days
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What happens next
- Your email program opens a prepared request with your domain. Add your name and send it.
- We get back to you and discuss with you or your IT partner which records (SPF, DKIM, DMARC) are needed and what the setup costs.
- The setup needs access to the DNS management of your domain. We agree the handover with you – please never send us passwords by email.
- If you wish, DomainRadar then monitors your domain continuously and alerts you when something changes.
No email program opens? Write to us with your domain at info@domainradar.ch.
Good to know: this protection stops anyone forging exactly your domain. Against look-alike addresses, always confirm payment changes by phone.
All checks
Quick check based on public DNS records. Continuous monitoring also checks certificates, blocklists and your DMARC reports.
Your result as a PDF
As a report to keep or to forward to your IT.
Free and without an account: you confirm your address by email and can unsubscribe at any time.
What this means for small businesses
Most SMEs send far fewer than 5,000 emails a day. For them, the strict rules aren’t a hard requirement. But Gmail and Outlook still filter emails without SPF, DKIM and DMARC more strictly. Then your quote lands in spam instead of with the customer.
Watch out for services that send in your name: newsletter tool, webshop, accounting or ticketing system. Each of these needs to be set up for your domain, otherwise their emails look like forgeries to Gmail and Outlook.
Test email to Gmail
Send an email to a private Gmail account and choose “Show original” from that email’s menu. Gmail then shows whether SPF, DKIM and DMARC passed.
What to do
- Read the non-delivery notice and note the code.
- Check your domain’s SPF, DKIM and DMARC, for example with the free check.
- List all services that send in your name and set up SPF and DKIM for each one.
- Check whether your mail server or domain is on a blocklist.
Frequently asked questions
Do Gmail’s and Outlook’s sender rules also apply to small businesses?
The strict requirements apply from 5,000 emails a day to Gmail or private Outlook addresses. But Google’s basic rules (SPF or DKIM, a valid name in the DNS, an encrypted connection) apply to all senders, and without DMARC emails are more likely to be filtered out.
What does the error message 550 5.7.515 mean?
Microsoft rejected the email because the sender domain didn’t meet the required authentication. Check your domain’s SPF, DKIM and DMARC, and the service the email was sent through.
Why does the same email arrive at one recipient but not another?
Every provider filters differently. Gmail and Outlook check especially strictly whether the sender can identify itself, and also factor in blocklists and their users’ spam reports. That’s why an email can arrive at one company and land in spam at Gmail.
How DomainRadar helps
DomainRadar continuously monitors your domain’s SPF, DKIM and DMARC, checks blocklists daily, and shows in the DMARC reports which services send in your name and whether Gmail, Outlook and other recipients recognise them as genuine. If something changes, you get an alert by email.
The domain check stays free. Afterwards you test DomainRadar with all features for 14 days – no credit card, the trial ends on its own.