Send & Receive Issues

How to Fix Microsoft Block List S3140 Email Delivery Errors

Elise Williams Sep 28, 2026 869 views

Question details

Users need to resolve email delivery failures where messages sent to Microsoft domains (Outlook, Hotmail, Live) are bounced back due to the sending IP being blacklisted.

How to Fix Microsoft Block List S3140 Email Delivery Errors

Product: Mailchimp / Email Service Provider
Device & OS: not provided
Scenario: Sending email campaigns or individual messages from a third-party email provider or Mailchimp to recipients using Microsoft email services.
Observed behavior: Emails fail to deliver and bounce back with a 550 5.7.1 error message stating that the sending IP address is on Microsoft's S3140 block list, despite public domain blacklist checks showing a clean reputation.

Before you start

Ensure you have access to your domain's DNS settings to verify your email authentication records and locate the exact bounced IP address from the 550 5.7.1 error message.

Solution 1Recommended

Request IP Delisting from Microsoft Sender Support

The most direct method to resolve the S3140 block is to submit an official delisting request to Microsoft's internal anti-spam team.

Microsoft maintains its own internal proprietary block lists based on network spam heuristics and sender reputation. Even if public blacklists show your domain is clean, you must go through Microsoft's specific channels to remove your sending IP from their S3140 list.

Identify the blocked IP address

Open the Non-Delivery Report (NDR) or bounce-back email you received. Locate the specific IP address mentioned in the 550 5.7.1 S3140 error code.

Visit the Delist Portal

Navigate to the official Microsoft Office 365 Anti-Spam IP Delist Portal in your web browser.

Submit your information

Enter the blocked IP address and an email address where Microsoft support can reach you. Complete the CAPTCHA and click Submit.

Confirm and wait for mitigation

Check your email for a verification link from Microsoft. Click it to confirm your request. Microsoft will review the IP reputation and typically respond within 24 hours regarding the delisting status.

Request IP Delisting from Microsoft Sender Support

Processing Time: Delisting is not always immediate. It can take up to 24 hours for changes to propagate across Microsoft's global email servers once the request is approved.

Solution 2

Contact Your Email Service Provider (Mailchimp)

If you are using a shared IP address provided by an email marketing service like Mailchimp, they must handle the delisting process.

Solution 3

Verify SPF, DKIM, and DMARC Configuration

Properly configuring your email authentication records is critical for building domain reputation and passing Microsoft's strict incoming spam filters.

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Frequently Asked Questions

Why is my IP blocked by Microsoft if public blacklists say my domain is clean?

Microsoft maintains its own internal, proprietary block lists (like the S3140 list) based on its unique network spam heuristics and user complaint data. Public blacklist checks do not query Microsoft's internal database, which is why your domain might appear clean publicly but still be blocked by Outlook or Hotmail.

What does the 550 5.7.1 error code mean in Outlook or Hotmail?

The 550 5.7.1 code is a standard Non-Delivery Report (NDR) indicating that the recipient's mail server rejected your message. In the context of Microsoft email services, this is often due to strict security policies, spam filters, or sender IP reputation issues.

How long does it take for Microsoft to delist an IP address?

Once you submit a delisting request via the Microsoft Anti-Spam IP Delist Portal and it is approved, it typically takes anywhere from a few hours up to 24 hours for their support team to mitigate the block across all their global servers.

Outlook & Email Problems