How to fix mail delivery error?

Question

How to fix mail delivery error?

Tineke K 25

When I send email from one of our domain names to Hotmail, Live or Outlook addresses I get an error as a reply: Mail delivery failed: returning message to sender After filling out the form at https://olcsupport.office.com/ the report says "Nothing was detected to prevent your mail from reaching Outlook.com customers", but the problem isn't solved. If the server apparantly isn't on a blacklist, but at least 4 domains get this error when mailing, what could be the problem?

0 comments

Answer accepted by question author

Kudos-Ng 15,055 Microsoft External Staff Moderator

Hi Tineke K,

Thank you for posting your question in the Microsoft Q&A forum.

To better assist you with troubleshooting this mail delivery error, could you please provide some additional details?

Were your domains working fine previously and this issue just started recently, or is this the first time you are setting up these domains and encountering this issue?
Is your environment Exchange Online (Microsoft 365) or Exchange Server (On-Premises)? Depending on your environment, you can run message tracking to view the full NDR message and detailed delivery status:
- For Exchange Online: Message trace in the modern Exchange admin center
- For Exchange Server: Message tracking

Additionally, based on my research, I found a document Microsoft Blacklist - Hetzner Docs that mentions when you receive this message, you should reply to the email to escalate the situation for further review by a human.

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Note: Microsoft is providing this information as a convenience to you. These sites are not controlled by Microsoft, and Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please ensure that you fully understand the risks before using any suggestions from the above link.

As a forum moderator, I do not have access to internal databases to confirm whether the above insight is officially verified by Microsoft.

I would appreciate it if you could share any updates.

Update information:

The original poster has confirmed that the useful tip is to reply directly to the initial automated email to request an IP review and delisting from the blocklist (3150), which will then reach a human agent. Once again, thank you to the OP for sharing this valuable insight based on real-world experience.

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Tineke K 25 Reputation points

2026-02-10T15:17:17.1866667+00:00

Hi @Kudos-Ng
The domains are active since a few years, the server is more than 10 years active. Untill a week ago there were no problems.
The Exchange-environment isn't from the sender but the receiver.

With kind regards,
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-11T09:27:34.69+00:00
Hi Tineke K,

Thank you for the quick follow-up.

The reason I asked about your sending environment is to determine the best troubleshooting path for the "IP reputation" issue mentioned by kagiyama yutaka.

If you are using an On-Premises Server (or Dedicated Server): You are directly responsible for your Sending IP's reputation. Since your server has been active for 10 years, it is possible that your specific IP address was recently flagged by Microsoft's consumer filters (Hotmail/Outlook.com), even if it is not on a public blacklist.

If you were using Exchange Online (Microsoft 365): The sending IPs are managed by Microsoft, so individual IP reputation is rarely the cause for a specific tenant.

Given your description (server active > 10 years), it strongly suggests you are sending from your own infrastructure.

For more recommendations:

Since the error is "Mail delivery failed" generic message, and assuming you manage your own server:

1) Check Microsoft SNDS: You can try to register your sending IP with Microsoft Smart Network Data Services (SNDS) to see if Outlook.com perceives "low-grade signal" or spam traffic from your IP.

2) Review Message tracking logs: On your sending server, identifying the exact NDR code (e.g., 550 5.7.1, S3150) from the "Mail delivery failed" bounce message is crucial to confirm if this is indeed a reputation block.

Also, I noticed you mentioned the support form returned "Nothing was detected." As mentioned earlier: Did you try replying to that automated email?

Usually, the first response is automated.

If you reply to that "Nothing was detected" email with the requested information, it often escalates the ticket to a human agent who can manually review and delist the IP.

Best regards,
Tineke K 25 Reputation points

2026-02-11T19:33:10.88+00:00

Thank you for the tips @Kudos-Ng
I registered the ip address with the SNDS. So far the ip status says "All of the specified IPs have normal status."

The full error in the mail is

host hotmail-com.olc.protection.outlook.com [52.101.11.16] SMTP error from remote mail server after pipelined MAIL FROM:******@xxxx.com SIZE=1473911: 550 5.7.1 Unfortunately, messages from [xx.xx.xx.xx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.asp[truncated]

The link ends up in an internal server error (500 code)

As you advised I replied to the mail. I noticed it is sent to Outlook.com Deliverability Support Team. Can they fix the problem for all Microsoft domains or only Outlook?
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-12T08:16:52.8+00:00

Hi Tineke K,

Thank you for the update on the situation.

Based on the full error details, it appears that the IP provided by your Internet service provider is indeed listed on the Outlook.com (consumer) block list. As a result, all domains hosted on this server/IP will be affected, not just a single domain.

As mentioned in the error message, you can contact your Internet service provider to work together on resolving this. If you are the Internet service provider and you’ve already replied to the message sent from olcsupport.office.com, then you should receive a response from a human agent after some time. They may request additional information to review the IP and proceed with delisting if it meets the required criteria.

To clarify, because this request falls under Outlook.com (consumer) support, their scope focuses on issues involving sending or receiving mail to Hotmail, Live, and Outlook consumer accounts, not Microsoft 365 business recipients.

I hope the information above helps, and I’d truly appreciate it if you could update the result of the steps you take here, so that others facing a similar issue can also benefit from your insights.
Tineke K 25 Reputation points

2026-02-12T09:58:26.3233333+00:00

Hi @Kudos-Ng
So far it looks like the mysterious problem is solved. Email to Hotmail addresses have been delivered without any error.
I think the action of Sheikh from Outlook.com Deliverability Support did solve the case:

We have implemented mitigation for your IP and this process may take 24 - 48 hours to replicate completely throughout our system.

So kudos for Sheikh, but also a big thank you for helping me out with your tips.
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-12T10:02:00.8666667+00:00

Hi Tineke K,

Thank you for your quick follow‑up.

I’m really glad I could support you and offer a helpful tip for this kind of issue.

If you find my answer useful, I’d truly appreciate it if you could consider accepting my answer so it can be highlighted as this helps other community users discover and benefit from the solution more easily.

Thanks so much again for engaging.
Tineke K 25 Reputation points

2026-02-12T14:38:10.49+00:00

I would like to accept one of the answers @Kudos-Ng but I can't select the one with the solution. Only your first reply
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-12T14:42:27.21+00:00

Hi Tineke K,

Thank you for your question.

You can accept my first answer, all subsequent comments will remain attached to it, making it easier for others to read the entire discussion in one place.
Tineke K 25 Reputation points

2026-02-12T14:52:57.85+00:00

That sounds misleading. It's not the answer to the problem. Sounds like a bug in the software. You shouldn't accept the start of a dialogue, but the real answer.
But I will do so to help you 😊
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-13T06:18:00.0866667+00:00

Hi Tineke K,

Thank you so much for your kind response.

I’m really glad to hear that the suggestions I provided were helpful for your issue, and I appreciate your enthusiastic support even more.

Regarding the information you shared, I’ll update my first answer to include the useful tip about replying to the automated email in order to reach a human agent, so that other users can easily see it.

Have a nice day!

1 additional answer

Your answer

Tineke K 25 Reputation points

2026-02-10T15:17:17.1866667+00:00

Hi @Kudos-Ng
The domains are active since a few years, the server is more than 10 years active. Untill a week ago there were no problems.
The Exchange-environment isn't from the sender but the receiver.

With kind regards,
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-11T09:27:34.69+00:00

Hi Tineke K,

Thank you for the quick follow-up.

The reason I asked about your sending environment is to determine the best troubleshooting path for the "IP reputation" issue mentioned by kagiyama yutaka.

If you are using an On-Premises Server (or Dedicated Server): You are directly responsible for your Sending IP's reputation. Since your server has been active for 10 years, it is possible that your specific IP address was recently flagged by Microsoft's consumer filters (Hotmail/Outlook.com), even if it is not on a public blacklist.

If you were using Exchange Online (Microsoft 365): The sending IPs are managed by Microsoft, so individual IP reputation is rarely the cause for a specific tenant.

Given your description (server active > 10 years), it strongly suggests you are sending from your own infrastructure.

For more recommendations:

Since the error is "Mail delivery failed" generic message, and assuming you manage your own server:

1) Check Microsoft SNDS: You can try to register your sending IP with Microsoft Smart Network Data Services (SNDS) to see if Outlook.com perceives "low-grade signal" or spam traffic from your IP.

2) Review Message tracking logs: On your sending server, identifying the exact NDR code (e.g., 550 5.7.1, S3150) from the "Mail delivery failed" bounce message is crucial to confirm if this is indeed a reputation block.

Also, I noticed you mentioned the support form returned "Nothing was detected." As mentioned earlier: Did you try replying to that automated email?

Usually, the first response is automated.

If you reply to that "Nothing was detected" email with the requested information, it often escalates the ticket to a human agent who can manually review and delist the IP.

Best regards,
Tineke K 25 Reputation points

2026-02-11T19:33:10.88+00:00

Thank you for the tips @Kudos-Ng
I registered the ip address with the SNDS. So far the ip status says "All of the specified IPs have normal status."

The full error in the mail is

host hotmail-com.olc.protection.outlook.com [52.101.11.16] SMTP error from remote mail server after pipelined MAIL FROM:******@xxxx.com SIZE=1473911: 550 5.7.1 Unfortunately, messages from [xx.xx.xx.xx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.asp[truncated]

The link ends up in an internal server error (500 code)

As you advised I replied to the mail. I noticed it is sent to Outlook.com Deliverability Support Team. Can they fix the problem for all Microsoft domains or only Outlook?
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-12T08:16:52.8+00:00

Hi Tineke K,

Thank you for the update on the situation.

Based on the full error details, it appears that the IP provided by your Internet service provider is indeed listed on the Outlook.com (consumer) block list. As a result, all domains hosted on this server/IP will be affected, not just a single domain.

As mentioned in the error message, you can contact your Internet service provider to work together on resolving this. If you are the Internet service provider and you’ve already replied to the message sent from olcsupport.office.com, then you should receive a response from a human agent after some time. They may request additional information to review the IP and proceed with delisting if it meets the required criteria.

To clarify, because this request falls under Outlook.com (consumer) support, their scope focuses on issues involving sending or receiving mail to Hotmail, Live, and Outlook consumer accounts, not Microsoft 365 business recipients.

I hope the information above helps, and I’d truly appreciate it if you could update the result of the steps you take here, so that others facing a similar issue can also benefit from your insights.
Tineke K 25 Reputation points

2026-02-12T09:58:26.3233333+00:00

Hi @Kudos-Ng
So far it looks like the mysterious problem is solved. Email to Hotmail addresses have been delivered without any error.
I think the action of Sheikh from Outlook.com Deliverability Support did solve the case:

We have implemented mitigation for your IP and this process may take 24 - 48 hours to replicate completely throughout our system.

So kudos for Sheikh, but also a big thank you for helping me out with your tips.
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-12T10:02:00.8666667+00:00

Hi Tineke K,

Thank you for your quick follow‑up.

I’m really glad I could support you and offer a helpful tip for this kind of issue.

If you find my answer useful, I’d truly appreciate it if you could consider accepting my answer so it can be highlighted as this helps other community users discover and benefit from the solution more easily.

Thanks so much again for engaging.
Tineke K 25 Reputation points

2026-02-12T14:38:10.49+00:00

I would like to accept one of the answers @Kudos-Ng but I can't select the one with the solution. Only your first reply
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-12T14:42:27.21+00:00

Hi Tineke K,

Thank you for your question.

You can accept my first answer, all subsequent comments will remain attached to it, making it easier for others to read the entire discussion in one place.
Tineke K 25 Reputation points

2026-02-12T14:52:57.85+00:00

That sounds misleading. It's not the answer to the problem. Sounds like a bug in the software. You shouldn't accept the start of a dialogue, but the real answer.
But I will do so to help you 😊
Kudos-Ng 15,055 Reputation points Microsoft External Staff Moderator

2026-02-13T06:18:00.0866667+00:00

Hi Tineke K,

Thank you so much for your kind response.

I’m really glad to hear that the suggestions I provided were helpful for your issue, and I appreciate your enthusiastic support even more.

Regarding the information you shared, I’ll update my first answer to include the useful tip about replying to the automated email in order to reach a human agent, so that other users can easily see it.

Have a nice day!

Answer 1

kagiyama yutaka 5,575

outlook isn’t rejecting ur domains… it’s tripping on the IP’s low‑grade signal‑noise, and that layer sits before any blacklist, so every domain behind that IP gets sunk the same.

Tineke K 25 Reputation points

2026-02-10T15:12:28.3466667+00:00

How can I test that ip's low-grade signal-noise you're refering to @kagiyama yutaka ?
kagiyama yutaka 5,575 Reputation points

2026-02-19T08:11:05.8033333+00:00

I mean best shot is sendin from that IP to a few neutral inboxes n peek the raw headers and see how long the hop hangs or if TLS gets stepped down… that’s kinda the only way u can feel that noise.