How to resolve Microsoft deliverability issues and IP blocks when using Salesforce Marketing Cloud?
Michael Ko
Co-founder & CEO, Suped
Published 3 Jul 2025
Updated 19 Aug 2025
7 min read
Dealing with email deliverability issues to Microsoft domains, especially when using Salesforce Marketing Cloud, can be incredibly frustrating. One common scenario involves persistent IP blocks, often indicated by a 5.7.1 bounce message. My experience shows that while the error clearly states an IP is on a blocklist, Microsoft's initial responses might sometimes claim there's no problem with the IP. This situation demands a persistent and strategic approach to resolution.
It's important to understand the nuances of these blocks and how to effectively communicate with both Microsoft and Salesforce support teams. This guide will walk you through diagnosing, troubleshooting, and ultimately resolving Microsoft deliverability issues and IP blocklists (or blacklists) when sending emails via Salesforce Marketing Cloud.
Understanding Microsoft's filtering and bounce codes
Microsoft, particularly its consumer mailboxes like Outlook and Hotmail, employs sophisticated filtering systems to protect its users from unwanted email. When you encounter a 5.7.1 (delivery not authorized) bounce message with an S3150 error code, it typically means your sending IP address, or part of its network, is on their internal blocklist (or blacklist). This block is usually reputation-based, indicating that Microsoft has detected patterns they deem undesirable.
The tricky part is that Microsoft's initial response to delisting requests can sometimes be an automated message stating no problem or that the IP doesn't qualify for mitigation. This doesn't mean your IP isn't blocked, but rather that Microsoft sees no issue with their decision to block it. It reflects their assessment of your sending practices. For a more detailed understanding of these challenges, review common email blocking issues with Microsoft domains.
To overcome this, you need to be highly persistent and ready to escalate. The key is to demonstrate that you've identified and rectified the underlying sending issues. If you are struggling with this, understanding how to deal with Microsoft Sender Support's automated responses is crucial.
Understanding the S3150 error code
When Microsoft returns a 5.7.1 error with the S3150 code, it signifies a specific type of IP-based block (or blacklist). The message typically states something like "messages from [IP address] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150)." This clearly points to a reputation issue on the sending IP, which in the context of Salesforce Marketing Cloud, means your dedicated IP is flagged. Microsoft's policies and guidelines (policies.aspx) also mention this, guiding senders to their support contact form.
Common causes of Microsoft IP blocks and reputation issues
IP blocks on Microsoft domains, particularly when using a dedicated IP with Salesforce Marketing Cloud, are almost always tied to sender reputation. This reputation is built over time based on various factors, including the quality of your mailing list, user engagement, complaint rates, and authentication practices.
Common culprits often involve sending too broadly to unengaged recipients, poor list hygiene leading to spam traps, or even issues within your email content. For example, some senders have reported blocks due to including insecure HTTP links in their email content, which Microsoft (and other ISPs) may flag. Understanding how email blacklists actually work can shed more light on these mechanisms. Additionally, issues can arise even during the IP warm-up process with Microsoft.
While you might be using a dedicated IP, if it's part of a shared pool initially, or if previous campaigns (even from other departments or business units within your organization) have caused reputation issues, it can lead to broad blocklists. This is why it's critical to identify the specific sending events or content that triggered the block.
Common problems
Poor list hygiene: Sending to old, unengaged, or invalid email addresses can lead to high bounce rates and spam trap hits.
Content issues: Spammy keywords, poor HTML, or insecure links (HTTP instead of HTTPS) can trigger filters.
Inconsistent sending volume: Sudden spikes in email volume can be perceived as suspicious, especially for a new or un-warmed IP.
Impact on deliverability
IP blocklist: Your Salesforce Marketing Cloud IP address gets added to a Microsoft internal blocklist, leading to bounce messages like the S3150 error.
Low inbox placement: Even if not completely blocked, emails may consistently land in the spam or junk folder.
Damaged sender reputation: A low reputation can affect deliverability across all ISPs, not just Microsoft.
Steps to diagnose and resolve issues
The first step is always to gather concrete evidence of the block. You'll need the raw bounce messages from your Salesforce Marketing Cloud data views. These messages contain the specific error codes and often point directly to the cause, such as the S3150 block. Salesforce support can help you retrieve these detailed bounce logs if you are unable to access them directly. You can find more information on diagnosing and fixing deliverability issues in Salesforce if needed.
Example of a 5.7.1 S3150 bounce message
5.7.1 (delivery not authorized) Unfortunately, messages from [128.245.XX.XXX] 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. [AM0EUR02FT021.eop-EUR02.prod.protection.outlook.com 2023-05-28T14:10:30.482Z 08DB5F3E6A88DDF0]
Once you have the bounce message, you or your IT administrator should submit a delisting request directly to Microsoft. It's crucial to use the correct form for consumer mailboxes. The form linked in many bounce messages, and often found on Microsoft's sender support page, is usually this specific form. Be prepared for initial denials, and keep replying to their emails with the bounce message and a clear request for escalation. This persistence is often what leads to a breakthrough. For more context on why Salesforce emails are blocked or refused, consult our detailed guide.
Simultaneously, lean heavily on Salesforce support. It's their responsibility as your ESP to help manage and resolve IP blocks. Provide them with the bounce messages and any communication you've had with Microsoft. Ask them to escalate the issue internally and to engage with Microsoft on your behalf, as they may have direct channels for resolving such blocklists. In some cases, Salesforce support's direct intervention has been more effective than client-initiated requests.
Persistence pays off
I've seen many cases where continuous follow-up with Microsoft, even after initial rejections, eventually leads to a resolution. They might first say your IP is fine, then that it doesn't qualify for mitigation, but if you persist and provide the clear bounce details (e.g., S3150 error code), they often acknowledge the block and work on mitigation. This mirrors insights from Spam Resource's guide on Microsoft OLC, which highlights the specific delisting forms and processes.
Proactive strategies for long-term deliverability
To prevent future deliverability issues and IP blocks (or blacklists), a proactive approach is essential. Maintaining excellent email list hygiene is paramount. Regularly remove unengaged subscribers, invalid addresses, and spam traps. Segment your audience to send relevant content to interested recipients, which naturally improves engagement and reduces complaints.
Strong email authentication is non-negotiable. Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Microsoft, like other major ISPs, increasingly relies on these standards to verify legitimate senders. Proper authentication builds trust and helps your emails reach the inbox. Our simple guide to DMARC, SPF, and DKIM can provide further assistance. You may also want to comply with Outlook's new sender requirements.
Finally, continuous monitoring of your sender reputation and deliverability metrics is vital. Tools like Microsoft's Sender Network Data Services (SNDS) and Junk Mail Reporting Program (JMRP) provide insights into how Microsoft views your sending IPs. Regular checks allow you to catch potential issues early and address them before they escalate into full IP blocklists.
List hygiene: Regularly clean your email lists to remove inactive subscribers and potential spam traps.
Engagement monitoring: Segment based on engagement and avoid mailing disengaged users.
Content quality: Ensure your email content is high-quality, relevant, and free of spam triggers, including mixed HTTP/HTTPS content.
Authentication: Properly configure SPF, DKIM, and DMARC for your sending domains.
IP warming: If using a new dedicated IP, follow a disciplined IP warming strategy.
Practice
Impact on Microsoft Deliverability
Send permission-based email
Reduces complaints and spam trap hits, directly improving sender reputation with Outlook.
Monitor SNDS/JMRP
Provides direct feedback from Microsoft about your IP's health and complaint rates, allowing for quick adjustments.
Verifies your sending identity, significantly reducing the likelihood of being marked as spam or blocklisted.
Views from the trenches
Best practices
Ensure you are obtaining raw bounce messages directly from Salesforce Marketing Cloud for accurate diagnosis.
Always use the correct Microsoft delisting form for consumer mailboxes; verify the URL to avoid confusion.
Work closely with Salesforce Marketing Cloud support, providing them with all details for their intervention and escalation.
Common pitfalls
Accepting an initial 'no problem' response from Microsoft without further persistence and escalation.
Failing to identify and fix the root cause of the block (e.g., poor list quality or insecure content) before requesting delisting.
Not regularly monitoring IP reputation and engagement metrics through tools like Microsoft SNDS.
Expert tips
Be persistent: Keep replying to Microsoft's support emails, even if they deny the block or mitigation. Provide concrete evidence like the S3150 error code to prove the issue.
Content review: Conduct deep dives into your email content, including all links and images, to ensure no insecure HTTP elements are present.
SFMC support escalation: Push Salesforce Marketing Cloud support to escalate the issue internally and with Microsoft; they have channels to expedite resolution.
Expert view
Expert from Email Geeks says when Microsoft replies that there is no problem, it often means the IP is blocked for a valid reason, and they see no issue with their decision. Persistence in asking for escalation is key.
2023-08-01 - Email Geeks
Marketer view
Marketer from Email Geeks says in their experience with Salesforce Marketing Cloud, issues often arose from sending images or links with HTTP instead of HTTPS, leading to IP blocks.
2023-08-01 - Email Geeks
Sustaining strong email delivery
Resolving Microsoft deliverability issues and IP blocks when using Salesforce Marketing Cloud requires a multifaceted approach. It's rarely a quick fix and often involves detailed troubleshooting, persistent communication, and a commitment to best practices. By understanding the common causes, effectively diagnosing bounce messages, and leveraging both Microsoft's and Salesforce's support channels, you can navigate these challenges.
Ultimately, a strong sender reputation built on consistent, permission-based sending, good list hygiene, and robust authentication is your best defense against blocklists. Continuous monitoring and a proactive stance will ensure your Salesforce Marketing Cloud emails consistently reach Microsoft inboxes.
When Microsoft returns a 5.7.1 error with the S3150 code, it signifies a specific type of IP-based block (or blacklist). The message typically states something like "messages from [IP address] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150)." This clearly points to a reputation issue on the sending IP, which in the context of Salesforce Marketing Cloud, means your dedicated IP is flagged. Microsoft's policies and guidelines (policies.aspx) also mention this, guiding senders to their support contact form.
IP blocklist: Your Salesforce Marketing Cloud IP address gets added to a Microsoft internal blocklist, leading to bounce messages like the S3150 error.
Outlook.
