Emails from our company domain blocked by Outlook.com — Block List S3150 — stuck in support loop

Question

Emails from our company domain blocked by Outlook.com — Block List S3150 — stuck in support loop

eunji lee 0

Hello,

We are a legitimate business sender (ohmckorea.com) whose outbound emails to Outlook.com/Hotmail.com recipients have been bouncing for several days, and we've been unable to get this resolved through the official support channels.

The error:

550 5.7.1 Unfortunately, messages from [1.244.192.62] weren't sent.
Please contact your Internet service provider since part of their network
is on our block list (S3150).

Sending IP: [Moderator note: Personally Identifiable Information removed]
Sending domain: ohmckorea.com
First occurred: 2026-08-26 (UTC)
Still occurring as of: 2026-08-31 (UTC)

What we've already tried:

Submitted a delisting request via the self-service portal (olcsupport.office.com). It was denied with "Not qualified for mitigation" (Service Request: 01M1AN5M6MFJ026MNZ9R5ATPJ5).
Replied to that denial asking for further review. We were redirected to general Microsoft 365 Technical Support.
That team redirected us to file a ticket via the Engage Center (Support for Business), but Outlook.com/Exchange Online Protection is not an available product option there, and we could not switch away from the "Support for Business" context.
Forwarded the original NDR to [[Moderator note: Personally Identifiable Information removed]] as instructed by Microsoft's own troubleshooting documentation. We received the same generic redirect back to Microsoft 365 Technical Support.

We appear to be stuck in a loop between teams that each say this isn't their area.

Additional context:

SPF record is correctly configured and includes our sending IP ([Moderator note: Personally Identifiable Information removed]).
We are in the process of adding a DMARC record, which was previously missing.
Our sending volume is low and consists only of legitimate one-to-one business correspondence (hotel/hospitality industry), not bulk or marketing email.

Could someone from Microsoft please help escalate this to the Outlook.com Deliverability Support team directly, or point us to a working channel? Happy to provide any additional information needed (full headers, sample messages, etc.).

Thank you.

Moved from Outlook | Web | Outlook.com | Account management, security, and privacy

0 comments

1 answer

Your answer

Answer 1

Dear eunji lee,

The S3150 error code confirms that your server's IP address has been blocked by the automated spam filtering systems for Outlook.com and Hotmail.

Below are the steps I recommend addressing the issue:

1.Submit a Delisting Request via Microsoft Sender Support

This is the most direct way to have your IP removed from the blocklist. Please submit a request through Microsoft's official Sender Support portal: https://sender.office.com

When submitting your request, include the following:

The blocked IP address.
The full Non-Delivery Report (NDR) or message headers showing the rejection.
Confirmation that the IP is not listed on any public blocklists.
A brief explanation of the corrective actions you’ve taken to improve your sender reputation.

Important: Be sure to complete the email verification step after submitting the form to finalize the process.

2.Check Your SPF, DKIM, and DMARC Configuration

Properly configured authentication records are essential for building and maintaining a strong sender reputation. Ensure your domain has valid SPF, DKIM, and DMARC records aligned with your sending setup.

SPF: For Microsoft 365, the recommended SPF record is v=spf1 include:spf.protection.outlook.com -all
DKIM: Confirm that your outgoing messages are signed with a valid DKIM.
DMARC: Check that your DMARC policy is configured correctly. At a minimum, setting it to p=none allows you to monitor authentication without affecting delivery.

You can check your current domain setup using tools such as: https://mxtoolbox.com

Please note: This link is shared by community members for your convenience. It points to a third-party site that is not managed or verified by Microsoft. We can’t guarantee the quality, safety, or suitability of any content or software found there. Please review carefully and make sure you understand any potential risks before using it.

3.Review Sending Infrastructure

Check whether your email traffic is being routed through on-premises servers or third-party relays. In these cases, the IP address evaluated by Microsoft may differ from your primary one, it’s important to identify the exact IP that was blocked.

4.Analyze Message Headers

Providing full message headers from a bounced email helps Microsoft pinpoint the exact cause of the block.
If you're unsure how to access message headers, follow this Microsoft guide: View internet message headers in Outlook - Microsoft Support
Then, paste the headers into the Message Header Analyzer tool.

Microsoft typically responds to delisting requests within 24 to 48 hours. If the issue persists beyond that timeframe or if it requires urgent resolution, I recommend reaching out to Microsoft Support directly via the Microsoft 365 Admin Center. For reference:  Get support | Microsoft Docs     

Alternatively, if you're using a third-party email service provider, you may also want to reach out to their support team, as they often have direct channels to help resolve sender reputation or IP blocking issues.

I hope this helps. Please feel free to reply below if you have any update or further concern.

Your understanding and co-operation are highly appreciated. Thanks for your precious time. Have a nice day!

Jade Ng 15,225 Reputation points Microsoft External Staff Moderator

2026-09-02T04:49:30.0966667+00:00

Dear eunji lee,

I hope everything is going well on your end.

I’m following up on the support thread we've been working through together. I wanted to check in and see if the information I shared in my previous answer helped clarify the issue you were experiencing.
eunji lee 0 Reputation points

2026-09-03T00:06:48.6533333+00:00

Hi, @Jade Ng

Thank you for checking in. Unfortunately, the issue is still not resolved, and I want to be upfront that I'm not very technical, so I'd really appreciate a plain explanation if possible.

Here is everything I've tried so far, and none of it has changed the outcome:

Checked the IP status at https://sender.office.com/ — it reports "This IP is currently not blocked in the system."

Emailed [Moderator note: Personally Identifiable Information removed] directly with the original NDR.

Submitted the delisting form at https://olcsupport.office.com/ — it was denied ("Not qualified for mitigation").

Added a DMARC record for our domain.

Despite all of this, we still get the same bounce when sending to Outlook.com/Hotmail.com recipients:

550 5.7.1 ... blocked using Blocklist 1 (S3150)

I've also identified the root cause on our side — a brief SMTP account compromise around Aug 15–17, which we've since remediated — and shared this in our support request as well.

Could you help explain, in simple terms, why none of these steps have worked, and what exactly we should do next? I want to make sure I'm not missing something obvious, but I honestly don't understand what else is expected of us at this point.

Thank you for your patience and continued help.
Hornblower409 14,425 Reputation points

2026-09-06T22:53:38.75+00:00

@eunji lee

One other thing to try - Check your reputation with Microsoft.

Validate your ownership of the sending address:

Go to the SNDS Home page: https://sendersupport.olc.protection.outlook.com/snds/

Sign in with any Microsoft account.

Choose [Request Access] and enter the sending IP address.

Once you have been validated - Pull some Reputation reports:

Go back to SNDS Home and choose [View Data].

You should see one or more validated IP addresses or ranges.

Click an IP address.

Prompt your favorite AI with something like this for more help:

How to read Reputation Reports from Microsoft Smart Network Data Services (SNDS)
Jade Ng 15,225 Reputation points Microsoft External Staff Moderator

2026-09-07T05:43:01.44+00:00

Dear eunji lee,

Thank you for your update!

In simple terms, the S3150 message means Microsoft's automated filtering systems have determined that mail from the sending IP has a reputation issue. Even though you've resolved the underlying compromise and implemented additional email security measures, reputation-based blocks are not always removed immediately, and the automated review process may determine that additional monitoring or investigation is required before mitigation can be granted.

I would also recommend reviewing the response provided by Hornblower409 and checking whether the suggested troubleshooting steps apply to your situation.

If the issue persists after reviewing that guidance, a support engineer will likely need to investigate the IP's deliverability status further, as there may not be any additional self-service actions available.

At this point, I recommend reaching out to Microsoft Support directly via the Microsoft 365 Admin Center.

For reference:  Get support | Microsoft Docs     

Alternatively, if you're using a third-party email service provider, you may also want to reach out to their support team, as they often have direct channels to help resolve sender reputation or IP blocking issues.

I truly appreciate your patience during this time and thank you for bearing with me.