If you are seeing deliverability issues at Microsoft, such as emails being rejected, delayed, or landing in spam, this article covers the most common causes and how to fix them.
Before you start
- Find your bounce details in Brevo. Bounce details include the error code returned by Microsoft's mail servers. To learn how to find them, check our dedicated article What are soft bounces and hard bounces in email?.
- Set up Microsoft SNDS if you use a dedicated IP. Smart Network Data Services (SNDS) is Microsoft's free tool for monitoring your IP reputation with Outlook, Hotmail, Live, and MSN. Since Brevo's IP ranges are registered under Brevo's name, Microsoft can only send the SNDS verification email to us, not to you, so you can't complete registration on your own. Contact our support team by creating a ticket from your account with your dedicated IP address, and we'll complete verification so you can access your SNDS data.
Find the right section
Use the tabs below to navigate to the section that matches your situation.
Use this table to match your symptom to its likely cause and find the right section.
| What you are seeing | Possible cause | Go to |
|---|---|---|
| Emails rejected, not delivered at all, for personal Outlook, Hotmail, Live, or MSN addresses | Domain authentication failure or an active block | Domain authentication failure or Blocked by Microsoft (Personal Outlook, Hotmail, Live, or MSN addresses) |
| Emails rejected only for a business address hosted on Microsoft 365 | A block at the Exchange Online layer, separate from personal Microsoft addresses | Blocked by Microsoft (Microsoft 365 business addresses) |
| The bounce message names a specific blocklist, such as Spamhaus | Your IP is listed on an external reputation blocklist that Microsoft checks | Blocked by an external blocklist |
| Emails delayed but eventually delivered | Rate limiting after a volume increase | Delivery delays and rate limiting |
| Emails delivered but going to spam | Content filtering or a reputation issue | Emails landing in spam |
| You cannot tell which contacts are marking your emails as junk | You are not enrolled in Microsoft's complaint feedback program (JMRP) | No visibility into complaints |
Use this table to match your error code to its likely cause. Some codes are broad and can signal more than one issue. Always read the full bounce message text alongside the code.
| Error code | Possible cause | Go to |
|---|---|---|
550 5.7.515 |
Your sending domain does not meet Microsoft's authentication requirement | Domain authentication failure |
550 5.7.1 mentioning S3150
|
Your IP is blocked at the Outlook, Hotmail, Live, or MSN consumer level | Blocked by Microsoft (Personal Outlook, Hotmail, Live, or MSN addresses) |
550 5.7.606 to 5.7.649
|
Your IP is on Microsoft 365's blocked senders list | Blocked by Microsoft (Microsoft 365 business addresses) |
550 5.7.1 naming an external blocklist |
Your IP is listed on a third-party reputation blocklist | Blocked by an external blocklist |
|
Microsoft is temporarily rate limiting or deferring your emails | Delivery delays and rate limiting |
550 5.7.0 |
Your email content was rejected by Microsoft's filters | Emails landing in spam |
Domain authentication failure
Domain authentication is how Microsoft checks that your email was genuinely sent by you, and not by someone pretending to be you. When domain authentication fails, Microsoft rejects the email before it reaches the inbox. It is the most common cause of delivery failures to Microsoft addresses and should always be checked first.
Signs of domain authentication failure
Common signs of domain authentication failure are:
- Bounce error
550 5.7.515, with a message stating that your sending domain does not meet the required authentication level. - All or nearly all your emails to Outlook, Hotmail, or Live addresses are failing, not just one campaign.
Why this happens
-
A DKIM, SPF, or DMARC record is missing or incorrectly configured on your domain.
💡 Good to knowSPF is only required if you are on a dedicated IP. - The domain in your "From" address does not match the domain your DKIM record is set up for.
- You are sending from multiple platforms with conflicting DNS records.
How to check your domain authentication
To check your domain authentication, review the headers of a delivered email:
- In Outlook, open a delivered email sent from your Brevo account.
- Click the three-dot menu icon, select View, then select View message source.
- Find the Authentication-Results header.
- Confirm that you see spf=pass, dkim=pass, and dmarc=pass.
How to fix domain authentication issues
➡️ To learn how to fix domain authentication issues, check our dedicated article Troubleshooting - Domain authentication.
Blocked by Microsoft
Microsoft filters personal Outlook, Hotmail, Live, and MSN addresses and Microsoft 365 business addresses through two separate systems. A reply stating that no block was detected from one system does not mean your IP is clear on the other, so start by identifying which one applies to your case:
| System | Covers | Delisting portal |
|---|---|---|
| OLC, Outlook.com Consumer | Personal Outlook, Hotmail, Live, or MSN addresses | olcsupport.office.com |
| Exchange Online, part of Microsoft 365 | Microsoft 365 business addresses | sender.office.com |
Select the tab that matches your case for the full signs, causes, and fix:
Microsoft filters personal Outlook, Hotmail, Live, and MSN addresses through a system called Outlook.com Consumer (OLC), more commonly referred to simply as Outlook.com. If your IP has accumulated too many spam complaints or hit a spam trap, Microsoft blocks it at this level, and every email sent to these addresses is rejected.
Signs of a block
Bounce error 550 5.7.1 mentioning S3150, stating that messages from your IP were not sent because part of your network is on Microsoft's blocklist.
Why this happens
- Your IP has accumulated too many spam complaints from Microsoft recipients.
- Your emails have hit a spam trap address, which usually points to a list hygiene problem.
How to fix a block
To fix a block, you need to submit a delisting request to Microsoft:
- [Recommended] Start by checking your IP status in SNDS:
- Go to SNDS and log in to your Microsoft account.
- Select View IP Status.
-
Check the Filter result for your IP: green means healthy, yellow means to investigate, and red means the IP is actively being filtered as spam.
- Go to olcsupport.office.com and submit a delisting request. If you checked your IP status in SNDS, include your filter result in your request. This shows Microsoft that you have already diagnosed the issue.
- Microsoft's first response is automated and comes back as one of three outcomes:
- Your IP was mitigated,
- No block was detected, or
- Your IP is not eligible for mitigation
If you get either of the last two but bounces continue, reply directly to that email. Replying is what connects you to a live support agent who can take the case further.
- At the same time, clean your contact list and reduce your sending volume to address the root cause.
Business domains hosted on Microsoft 365 are filtered by Exchange Online, a separate system from personal Outlook, Hotmail, Live, and MSN addresses. A block here does not always show up when you check the OLC delisting portal, and the reverse is also true.
Signs of a block
- Bounce error
550 5.7.606to5.7.649, stating that your sending IP is banned. - The issue affects business addresses hosted on Microsoft 365 but not personal outlook.com or hotmail.com addresses.
Why this happens
- Your IP reputation at the Exchange Online layer has degraded, separately from OLC.
- An individual business's own spam policy is blocking your domain or IP. In this case, the block is set by that business and can only be removed by their IT team.
How to fix a block
5.7.511
instead, Microsoft has flagged this specific IP for
manual investigation rather than an automated
check, so the delist portal cannot be used. Forward the full
bounce message to
delist@microsoft.com
instead, and expect a response within about 48 hours.
- Go to the Office 365 Anti-Spam IP Delist Portal.
- Enter your email address and the IP address shown in the bounce error message.
- Submit the request.
- Check your inbox for a confirmation email from Microsoft and click the link to confirm your email address.
- In the delist portal, select Delist IP.
Blocked by an external blocklist
Microsoft's rejection message sometimes names a specific third-party reputation blocklist, such as Spamhaus. This means your IP has been listed by that blocklist provider, not by Microsoft itself.
Signs of an external blocklist reference
Bounce error 550 5.7.1, with the message naming a specific blocklist provider.
How to fix an external blocklist reference
➡️ To fix an external blocklist reference, check our dedicated article FAQs - Why is my IP address or domain blocklisted?.
Delivery delays and rate limiting
If you send a large number of emails too quickly, Microsoft temporarily delays delivery. These are temporary holds, not permanent blocks. Microsoft retries the emails automatically, so there is no need to resend them manually.
Signs of rate limiting
- Bounce error
451 4.7.650, stating that your mail server has exceeded the rate limit. - Bounce error
421 4.7.0, stating that messages from your IP are temporarily deferred. - A large send is partially delivered, with a significant percentage deferred.
Why this happens
- You sent a significantly larger volume than in previous days without warming up gradually.
- You are using a new IP or domain with no established sending history.
What to do to avoid rate limiting
- Spread large sends across multiple hours or days rather than sending everything at once.
- If you are using a new dedicated IP, warm it up first. To learn more, check our dedicated article Warm up your dedicated IP (manual warm-up) or Warm up your dedicated IP (automatic warm-up).
- If rate limiting continues despite low volume, it may signal a reputation issue. Dedicated IP clients can investigate further using SNDS.
Emails landing in spam
If your emails are being delivered but going to the Junk folder, the cause is usually either your content or your sender reputation. Every message that Microsoft scans is stamped with a Spam Confidence Level, or SCL, which you can use to tell the two apart.
Why this happens
- Your complaint rate has exceeded a healthy level for your sending IP.
- You are sending to old, unengaged, or purchased contact lists.
- Your email content contains elements Microsoft associates with spam: URL shorteners, excessive promotional language, or misleading subject lines.
- Your emails are made up mostly of images with little readable text.
How to check the Spam Confidence Level
- In Outlook, open the email in question.
- Click the three-dot menu icon, select View, then select View message source.
-
Find the X-Forefront-Antispam-Report header and locate the SCL value inside it.
SCL value Meaning -1 Filtering was bypassed for a trusted or safe-listed sender 0 to 1 Not considered spam, delivered to the inbox 5 to 6 Considered spam, moved to the Junk folder 7 to 9 Considered high confidence spam, aggressively filtered
If the SCL is high alongside a passing authentication result, content or reputation is the likely cause. If the SCL is high alongside a failing authentication result, fix your authentication first, since it is likely the root cause.
What to do to avoid spam placement
- Remove URL shorteners from your emails and set up a branded subdomain instead, so your tracking and image links use your own domain rather than Brevo's default one. To learn more, check our dedicated article Set up your domain in Brevo.
- Review your subject lines and "From" name so they accurately describe the email content.
- Balance the image-to-text ratio in your emails. Emails made up mostly of images with little readable text are harder for spam filters to analyze, which can make them look more like spam, and if images fail to load, readers see a mostly blank email.
- Send only to engaged contacts who have opened or clicked one of your emails recently, and move unengaged contacts to a separate list.
➡️ For more guidance on rebuilding your sender reputation, check our dedicated article Best practices for email deliverability.
No visibility into complaints
The Junk Mail Reporting Program, or JMRP, is Microsoft's complaint feedback loop for dedicated IP senders. When a Microsoft recipient marks your email as junk, JMRP sends a notification to your registered abuse mailbox. Without JMRP, these complaints stay invisible until your reputation has already degraded.
How to enroll in JMRP
- Go to Microsoft SNDS and sign in with the Microsoft account you use to manage your dedicated IP.
- Select Junk Mail Reporting Program from the menu.
- Register an abuse mailbox, for example abuse@yourdomain.com, to receive complaint reports.
Diagnostic tools
The following tools can help you diagnose and monitor deliverability to Microsoft addresses:
| Tool | URL | Who can use it | What you can do |
|---|---|---|---|
| Microsoft SNDS | substrate.office.com | Dedicated IP clients only | Monitor your IP's filter status, complaint data, and spam trap hits |
| JMRP | Via the SNDS portal, under Junk Mail Reporting Program | Dedicated IP clients only | Receive reports when recipients mark your email as junk |
| OLC delisting portal | olcsupport.office.com | All senders | Submit delisting requests for outlook.com, hotmail.com, live.com, or msn.com blocks |
| Office 365 delisting portal | sender.office.com | All senders | Submit delisting requests for Microsoft 365 business domain blocks |
| Outlook "View message source" | In Outlook, click the three-dot menu, select View, then View message source | All senders | View the raw message headers to confirm authentication results and the SCL value |
⏭️ What's next?
- Comply with Gmail, Yahoo, and Microsoft's requirements for email senders
- Troubleshooting - Domain authentication
- FAQs - Why is my IP address or domain blocklisted?
- Best practices for email deliverability
- Best practices for managing a dedicated IP
- Troubleshooting - Deliverability issues with Gmail
🤔 Have a question?
If you have a question, feel free to contact our support team by creating a ticket from your account. If you don't have an account yet, you can contact us here.
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