What should I do if Microsoft blocks my IP address during IP warming?

Key findings

• Bounce messages: The generic message of 'Blocked due to spam or sender reputation issue' from ESPs like Marketing Cloud is often insufficient. It's crucial to obtain the exact SMTP rejection code to understand the root cause of the block.
• Proactive steps: Microsoft (Outlook.com) can be sensitive during IP warming, even with careful volume increases. Proactive communication and requests for pre-emptive accommodation can help prevent issues.
• Immediate action: If blocked, immediately cease sending to Microsoft domains. Continuing to send while blocked can exacerbate the problem and prolong your remediation efforts.
• Engagement impact: Reviewing Microsoft-specific spam complaints and recipient engagement history on your list can reveal contributing factors to the block, as explored in our guide on navigating the Microsoft IP warming process.

Key considerations

• Identify bounce reason: Prioritize obtaining the precise bounce message from Microsoft. This critical information will dictate the appropriate next steps for remediation.
• Contact Microsoft: Once you have the bounce details, contact Microsoft support directly. Be prepared to provide your IP address, sending volumes, and a proposed re-warming plan, particularly for Microsoft-owned domains. We discuss this further in what to do if Microsoft blocks your IP.
• Adjust warming plan: Once unblocked, re-initiate sending to Microsoft domains slowly and gradually, even more cautiously than your initial warm-up. This may involve segmenting your list to focus on highly engaged Microsoft recipients first.
• Monitor closely: Continuously monitor your deliverability and bounce rates to Microsoft properties. Utilize Microsoft's Postmaster Tools (SNDS) for insights into your reputation and blocklist status.

Key opinions

• Bounce detail is key: ESPs' (like SFMC) generic bounce classifications are often unreliable. Marketers stress the importance of getting the actual SMTP rejection message to understand the true block reason.
• Stop sending to Microsoft: If you're blocked, continuing to send emails, especially to the blocked recipient domains, is counterproductive and can worsen the situation. It's better to pause and investigate.
• Microsoft's sensitivity: Microsoft's filtering can be very sensitive, even for carefully managed warm-ups. Sometimes, blocks can occur due to general blocklist issues affecting the larger IP block (which is common for shared IPs), as discussed in what happens when your IP gets blocklisted.
• Proactive outreach helps: Some marketers suggest proactive engagement with Microsoft, particularly for large-volume senders, to potentially gain 'pre-emptive accommodation' and smoother warming.

Key considerations

• Obtain full bounce reports: Insist on obtaining the precise SMTP rejection codes from your ESP. If your ESP doesn't provide this readily, investigate methods like SQL queries on bounce data views (if available, e.g., in Salesforce Marketing Cloud) to extract this information.
• Engage Microsoft Support: Once you have the specific bounce error, contact Microsoft support through their sender support forms. This is essential for initiating the delisting or mitigation process. Further details are available in how to resolve Microsoft deliverability issues.
• Review send practices: While awaiting Microsoft's response, review your email content, list acquisition practices, and overall sender reputation. Ensure compliance with Microsoft's sender requirements, as detailed in this guide to improving Outlook deliverability.

Key opinions

• Bounce code importance: Without the specific SMTP bounce code, diagnosing the true reason for a Microsoft block is incredibly difficult. This information is the first step in any remediation.
• Proactive engagement: Pre-emptive accommodation requests with Microsoft can significantly smooth the IP warming process for new IPs, especially for large sending volumes, as discussed in the Spamresource article on pre-emptive accommodation.
• Stop and resolve: When an IP is blocked, the immediate priority should be to stop sending to Microsoft domains and focus on resolving the block. Continued sending can lead to deeper reputation damage and prolong resolution time.
• Leverage Microsoft tools: Microsoft Postmaster Tools (SNDS) and their troubleshooting pages (e.g., postmaster.live.com) are essential resources for monitoring IP reputation and understanding specific block codes like S3150 or S775.

Key considerations

• Direct communication with Microsoft: Utilize Microsoft's sender support forms (e.g., the Microsoft form) to formally request delisting. Provide comprehensive details about your sending practices and remediation steps.
• Understand block types: Differentiate between hard blocks (e.g., S3150, indicating a blocklist entry) and temporary rate limiting (e.g., S775, due to IP reputation). Each requires a different response, as discussed in why your dedicated IP is blocked by Hotmail/Outlook.
• Gradual re-warming: If unblocked, restart sending to Microsoft domains very cautiously. This often means reducing volumes significantly and slowly increasing them again, focusing on highly engaged subscribers first.
• Authentication integrity: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Authentication failures can contribute to reputation issues and blocks, as detailed in a simple guide to DMARC, SPF, and DKIM.

Key findings

• Specific error codes: Microsoft's bounce messages provide specific codes (e.g., S3150 for blocklist, S775 for temporary rate limiting) that are crucial for diagnosing the issue. These are often linked to their troubleshooting page.
• Sender support forms: Microsoft provides a dedicated sender support form for senders to request delisting or inquire about IP reputation.
• Data views for bounces: ESPs like Salesforce Marketing Cloud offer data views (e.g., Bounce Data View) that can be queried to access detailed bounce information, including the exact rejection responses.
• Postmaster tools: Microsoft's Postmaster Tools (SNDS) provide insights into your IP and domain reputation, complaint rates, and blocklist status, which are essential for proactive monitoring and troubleshooting.

Key considerations

• Troubleshooting steps: Follow the official troubleshooting steps provided by Microsoft when an IP address is blocked. This typically involves reviewing the bounce message, checking SNDS, and submitting a delisting request if appropriate.
• Compliance with policies: Ensure your sending practices comply with Microsoft's guidelines for legitimate email senders. This includes proper list hygiene, managing complaints, and authentication (SPF, DKIM, DMARC), as explored in how to comply with Outlook's new sender requirements.
• Patience and persistence: Remediating an IP block, especially during warming, can take time. Microsoft documentation often implies that consistent good sending behavior and patient follow-up are necessary.