Emails not being delivered

Question

Hello,

I am using Outlook as my primary mailbox, where emails from my other email accounts are forwarded.

Unfortunately, since yesterday, emails from one of my work email accounts have stopped being delivered. Specifically, when I try to send an email from my domain @zcu.cz address to my Outlook address, the email is not delivered. It does not appear in the inbox or in the spam folder.

I have already checked the mailbox storage quota, and my Outlook mailbox is not full, so this does not appear to be a capacity-related issue.

Instead, the sender receives a bounce-back message with the following error:

• host outlook-[Moderator note: personal info removed]mx.microsoft [Moderator note: personal info removed] said: 550 5.7.1 Unfortunately, messages from [Moderator note: personal info removed] 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.aspx#errors [Name=Protocol Filter Agent][AGT=PFA][MxId=[Moderator note: personal info removed].eurprd02.prod.outlook.com 2026-02-16T09:16:21.230Z

It appears that the sending server IP address is blocked by Outlook/Microsoft.

Is there anything I can do on my side to resolve this issue?

Any guidance would be appreciated.

Thank you for your help.

Karel

Answer 1

Please note that our forum is a public platform, and we will modify your question to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data.

Hi Karel Řezáč

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

First, I’d like to clarify that this is a user‑to‑user support forum. Moderators, contributors, and external Microsoft employees participating here do not have access to backend systems, nor can we directly intervene in Microsoft product features. Our role is limited to providing technical guidance and sharing best practices based on reported issues, requests, or ideas.

May I ask your Outlook account is personal account (@outlook.com, @hotmail.com, @live.com) or business account (@yourdomainname)?

You can use the IP Delist Portal at https://sender.office.com is intended for business/enterprise recipients.

Or if you’re using personal email, the appropriate portal would be: https://olcsupport.office.com/

You may also find this article useful, as it includes additional suggestions and explanations about Microsoft delisting: https://bentonow.com/posts/microsoft-delisting-s3150-s3140

Note: This information is provided 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.

Additionally, if you are using Microsoft 365 services, you can ask your IT admin to submit a support ticket through the Admin Center. A Microsoft support agent will be able to assist you directly and provide more authoritative information.

Hope this helps. Let us know if you have any other questions.

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Answer 2

Sorry to hear you're having trouble.

I have two suggestions for you. First, as the error message said that part of your ISP is on the block list you should first contact them to see if there's been issues with spam as your messages are being blocked. If it is they should be able to take steps to fix the problem for you.

The second thing to try would be to contact your company's IT Department and let them know what's going on. They may also be able to get a workaround going for you so you can continue to send the messages.