block list (S3150)

Question

Hello everyone,

We are currently having unpleasant experiences with the block list (S3150) delivery error.

One of our customers has been migrated to an onprem mail system. The mailing works great, unless the recipient is an outlook.de, live.de or hotmail.de address. But even that works from time to time without anything being changed in the system.

Here is the error message:

**********@outlook.de: host eur.olc.protection.outlook.com[52.101.73.25]

said: 550 5.7.1 Unfortunately, messages from [193.238.162.75] weren't sent.

Please contact your Internet service provider since part of their network

is on our block list (S3150).

We ourselves are the provider of the IP address and have no problems with the address range for other customers when it comes to sending to personal Microsoft mailboxes. We have already been in contact with M365 support. He was able to confirm that the IP of our customer's mail server is on the block list. However, he could not tell us the reason for this and could not remove the entry either, as he is stuck in M365 support and does not have permissions on the block list of personal "non-business" Microsoft servers. We tried several times to contact the "correct" support for personal Outlook, but never got very far. We also submitted the approval form several times. The response was always that there was nothing to approve because the specified IP address was not on the block list.

What can I do to remedy this?

Thank you in advance for any support!

Edit:

SPF, DKIM and DMARC are configured correctly, which various tests have already confirmed. In addition, the mail server and the IP are dedicated to the customer. Only one domain sends via it.

We already posted this question here: Where the support redirected us to this page.

Here ist the link to the previous question in german:

https://answers.microsoft.com/de-de/outlook_com/forum/outlk_web-outtop_com-outsub_email/block-list-s3150/1828851e-3b8c-4b66-8bb3-7ac5f790caf7

Answer 1

Hi @Etienne Desweemer ,

Welcome to our forum!

Looks like your issue is more related to the Outlook.com delivery issue, please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client. Based on my research, i found that there're several factors may affect this issue, please refer to this article and check if your email complies with Microsoft policies and guidelines: Troubleshooting.

If your email complies with the policies and guidelines and you are still experiencing email delivery problems that are not addressed in the FAQ mentioned in the article above, please click [here] to contact support.

Hope to help you!

---

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

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.