My IP address seems to have been put on a block list

Question

I run a company from home and i cannot send emails out to any hotmail or outlook accounts. it was working fine up until yesterday. this is the message that im getting.

Subject: test Remote Server returned: '550 5.7.1 Unfortunately, messages from [[Moderator note: Personally Identifiable Information 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][[Moderator note: Personally Identifiable Information removed]]'

Answer 1

Dear @Matt Van Dyke,

The error message indicates that Microsoft’s Protocol Filter Agent has temporarily flagged your IP address on its internal blocklist. This frequently happens to home/small-business IP ranges due to a sudden change in mail traffic, missing DNS authentication records, or poor reputation flags associated with neighboring IPs in your provider’s network.

Here is how you can resolve this issue:

Step 1: Request IP Delisting from Microsoft

Depending on whether you are emailing consumer accounts (Hotmail/Outlook.com) or business accounts (Microsoft 365), submit a delisting request below:

• For Outlook.com / Hotmail.com recipients: Submit a ticket directly to Microsoft’s Outlook delivery support team using the Sender Information for Outlook.com Delivery Form. (Note: Set the issue title or case description to "IP blocked with error S3150 – Mitigation Request" to speed up processing.)
• For Microsoft 365 (Enterprise/Business) recipients: Use the official Microsoft 365 Anti-Spam IP Delist Portal to remove your IP from the blocked senders list.

Step 2: Verify Your Mail & DNS Configuration

Before or while submitting the request, ensure your sending environment meets Microsoft's requirements to prevent the IP from being re-blocked:

1. Reverse DNS (PTR Record): Ensure your IP has a valid reverse DNS (PTR) record that matches your mail domain. Home ISP connections often lack proper PTR alignment.
2. Email Authentication: Confirm you have valid SPF, DKIM, and DMARC DNS records published for your domain.
3. ISP / Relay Check: If your ISP assigns dynamic IP addresses or blocks port 25, consider relaying your outbound email through your domain’s authenticated SMTP service (e.g., Microsoft 365, Google Workspace, or a dedicated SMTP provider).

For detailed guidance on Office 365 blocking policy, see the official Microsoft documentation: Use the delist portal to remove yourself from the blocked senders list.

If you have any updates or further questions, please feel free to reply here, I’ll do my best to support you however I can. 

Thank you for your understanding.

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