Why are SFMC shared IPs having deliverability issues specifically with Microsoft domains?
Matthew Whittaker
Co-founder & CTO, Suped
Published 28 May 2025
Updated 16 Aug 2025
9 min read
It can be incredibly frustrating to see your emails from Salesforce Marketing Cloud (SFMC) perform well with some inbox providers, like Apple domains, but consistently run into deliverability roadblocks with Microsoft domains such as Outlook, Hotmail, and Live. Many senders using SFMC's shared IP addresses encounter these precise issues, experiencing high bounce rates or emails landing directly in spam folders. The typical bounce message often cites a 5.7.1 delivery not authorized error, indicating that part of the sending network is on a blocklist (or blacklist).
The core of the problem often lies in the nature of shared IP addresses. When you send emails from a shared IP, your sender reputation is intertwined with every other sender using that same IP. If even one or two other senders on the shared IP engage in questionable or spammy practices, it can quickly tarnish the reputation of the entire IP, leading to a blocklist (or blacklist) designation. This collective reputation then impacts your email deliverability, regardless of your individual sending quality.
Microsoft, in particular, maintains stringent spam filtering protocols. Their systems are highly sensitive to IP reputation and often outright block emails from IPs that show any signs of suspicious activity or a poor sending history. This aggressive approach aims to protect their users from unwanted mail, but it can disproportionately affect legitimate senders on shared IPs that are dragged down by others.
Successfully navigating these challenges requires a deep understanding of shared IP dynamics, Microsoft's specific policies, and strategic email sending practices. Troubleshooting email deliverability issues with Microsoft domains can feel like an uphill battle, especially when a third-party is managing the IP reputation.
The inherent risks of shared IP addresses
Shared IP addresses are used by multiple senders, meaning their collective sending behavior influences the IP's overall reputation. If one sender on the shared IP has poor list hygiene, sends unsolicited mail, or experiences high spam complaints, it negatively impacts every other sender using that same IP. This makes shared IPs inherently riskier for deliverability compared to a dedicated IP, where your reputation is solely based on your own sending practices.
This communal reputation directly affects how ISPs (Internet Service Providers), especially strict ones like Microsoft, perceive your email. If any portion of the shared IP's traffic triggers spam filters, the entire IP range can be flagged, leading to delivery failures for all legitimate senders. You might experience this as emails being filtered directly to the spam folder or, worse, completely blocked with hard bounces. This is a common challenge for those using Klaviyo shared IPs having Gmail deliverability issues too.
When your SFMC shared IP is impacted, you often see specific bounce codes like 5.7.1 Delivery Not Authorized accompanied by a reference to a blocklist (or blacklist) designation, such as S3140. This indicates that Microsoft's systems have identified the sending IP as problematic, either due to its general reputation or specific malicious activity originating from it. Understanding how email blacklists actually work is key to diagnosing these issues.
Microsoft's strict filtering and common bounce codes
The problem
SFMC shared IPs are susceptible to reputation damage from other users, leading to blocklists.
Microsoft's aggressive spam filtering often results in outright blocks with errors like 5.7.1 S3140.
As a sender, you have limited control over the shared IP's overall reputation, making direct resolution with Microsoft challenging.
Microsoft's email infrastructure, including Outlook.com and Hotmail, employs some of the most rigorous spam filtering rules in the industry. They are highly sensitive to sender reputation, especially for IP addresses that show inconsistent sending patterns or high complaint rates. The S3140 error often indicates that the IP address, or a range it belongs to, has been flagged due to suspected spam or malicious activity originating from that network segment. You can learn more about how Microsoft Online Services (OLC) handles deliverability in their ISP Deliverability Guide.
Because it's a shared IP address, Salesforce Marketing Cloud (SFMC) is the official owner of the IP. This means that direct delisting requests or reputation inquiries to Microsoft's Postmaster tools generally need to come from SFMC themselves. While you can certainly reach out to Salesforce for deliverability issues, their ability to permanently resolve issues on a shared IP is limited by the behavior of all users on that IP. They may advise you to purchase a dedicated IP if your sending volume is high enough.
Even if an IP block (or blacklist) is temporarily lifted, if other senders on the shared IP continue poor practices, the IP's reputation will quickly degrade again. This creates a frustrating cycle of deliverability problems where any 'fix' is short-lived. This is why Microsoft email deliverability can be poorer compared to Gmail and Yahoo, as their systems are less forgiving of shared IP issues.
Strategies for maintaining shared IP health
Shared IP
Reputation: Influenced by all senders on the same IP, making it volatile.
Control: Limited control over the overall IP reputation, reliant on ESP management.
Delisting: Requires the IP owner (e.g., Salesforce) to initiate the process with ISPs.
Dedicated IP
Reputation: Solely based on your sending behavior, offering stability.
Control: Full control over building and maintaining sender reputation.
Delisting: You can directly engage with Microsoft Postmaster tools.
Even with a shared IP, there are crucial steps you can take to improve your sender reputation and mitigate deliverability issues. First and foremost, focus on maintaining an exceptionally clean and engaged email list. Regularly remove inactive subscribers, hard bounces, and known spam trap addresses. Sending relevant, valuable content also minimizes spam complaints, which are a major red flag for ISPs like Microsoft. Low deliverability and open rates are often symptoms of poor list hygiene.
Consistency in your sending volume and frequency is another critical factor. Irregular spikes or drops in sending volume can appear suspicious to ISPs. Aim for a steady sending cadence to help build and maintain a predictable, positive sending profile for the shared IP. Salesforce also recommends consistent sending as a best practice for Microsoft domains. This steady behavior can help offset the negative impact of other users on the shared IP by demonstrating your consistent positive contributions to its reputation.
Always ensure your emails are properly authenticated with SPF, DKIM, and DMARC. These email authentication protocols verify that your emails are legitimate and prevent spoofing. While SFMC manages the IP, your domain's authentication is under your control and plays a significant role in deliverability. Even on shared IPs, strong authentication can boost your credibility with ISPs. For a straightforward explanation, refer to our guide on DMARC, SPF, and DKIM.
Long-term considerations and alternative solutions
Example authentication recordsDNS
SPF: v=spf1 include:spf.protection.outlook.com include:email.marketingcloud.com ~all
DKIM: (check your SFMC setup for specific DKIM keys)
DMARC: v=DMARC1; p=none; rua=mailto:reports@yourdomain.com; ruf=mailto:forensic@yourdomain.com; fo=1;
For senders with significant email volume or those sending highly critical transactional emails, relying solely on SFMC's shared IPs can lead to persistent deliverability challenges, particularly with sensitive recipients like Microsoft. If you find yourself repeatedly dealing with blocked bounces and struggling to get your messages delivered, it might be a sign that you need to grow out of a shared IP and consider a dedicated IP address.
A dedicated IP offers complete control over your sender reputation. While it requires an initial IP warming period to build a positive sending history, it ensures that your deliverability is not impacted by the actions of other senders. This level of control is invaluable for maintaining consistent inbox placement, especially with challenging ISPs. If moving away from SFMC is an option, explore other ESPs that align better with your sending volume and deliverability needs. Consider checking our expert guide to improving email deliverability.
Ultimately, the key to successful email deliverability with Microsoft domains, whether on shared or dedicated IPs, lies in a proactive approach to sender reputation. Continuously monitor your bounce rates, spam complaint rates, and inbox placement. Regularly review your email content and sending practices to ensure they align with best practices. For ongoing insights, remember to utilize why your emails are not delivering to Microsoft inboxes.
Views from the trenches
Best practices
Maintain a clean email list by regularly removing inactive or invalid addresses to reduce bounces and complaints.
Segment your audience and personalize content to increase engagement and avoid spam filter triggers.
Implement strong email authentication (SPF, DKIM, DMARC) for your sending domain.
Monitor your sender reputation and deliverability metrics, especially for Microsoft domains.
Gradually increase sending volume (IP warming) if you transition to a new IP or sending platform.
Ensure a clear and easy unsubscribe process to reduce spam complaints.
Common pitfalls
Ignoring bounce messages and continuing to send to problematic addresses, leading to further blocks.
Sending inconsistent volumes or frequencies, which can negatively impact IP reputation.
Failing to monitor shared IP blocklists, leaving issues unaddressed.
Expecting immediate fixes for shared IP blocks without addressing underlying sending practices.
Not aligning email content with recipient expectations, resulting in high complaint rates.
Underestimating Microsoft's strict filtering and hoping for the best with poor practices.
Expert tips
While SFMC shared IPs can be challenging, focus on optimizing everything within your control to be an exemplary sender on that shared IP.
For critical or high-volume sending, strongly advocate for a dedicated IP to gain full control over your sending reputation.
Leverage DMARC reports to identify authentication failures and improve your email security posture.
Proactively engage with your ESP's support, providing them with detailed bounce logs and metrics to help them address shared IP issues with ISPs.
Regularly check major blocklists (or blacklists) to see if your sending IP is listed and understand the specific reasons.
Consider transactional emails on separate IPs or different platforms if deliverability is paramount and your marketing emails are causing issues on shared IPs.
Marketer view
Marketer from Email Geeks says they have seen poor luck with SFMC shared IPs in general for their clients, especially at Microsoft.
2023-01-05 - Email Geeks
Marketer view
Marketer from Email Geeks says that they've found shared IPs are usually on a blocklist (or blacklist) when looking at the SMTP bounce reason.
2023-01-05 - Email Geeks
Finding your deliverability path forward
Dealing with SFMC shared IP deliverability issues, especially with Microsoft domains, can be a complex and ongoing challenge. The communal nature of shared IPs means your sending reputation is always at the mercy of others. While implementing best practices on your end can significantly help, ultimate control over the IP's standing with ISPs like Microsoft rests with the IP owner.
For critical email programs or high sending volumes, the move to a dedicated IP address offers a more stable and controllable solution. It allows you to build and maintain a reputation based solely on your own sending habits, providing a clearer path to consistent inbox delivery. Whether you stick with shared IPs or transition to dedicated ones, proactive monitoring and adherence to email best practices are non-negotiable for long-term deliverability success. Ultimately, preventing emails from having deliverability issues with Microsoft Outlook and Hotmail requires vigilance and adaptability.
Why are SFMC shared IPs having deliverability issues specifically with Microsoft domains? - Sender reputation - Email deliverability - Knowledge base - Suped
Apple domains, but consistently run into deliverability roadblocks with 
Microsoft domains such as Outlook, Hotmail, and Live. Many senders using SFMC's shared IP addresses encounter these precise issues, experiencing high bounce rates or emails landing directly in spam folders. The typical bounce message often cites a 5.7.1 delivery not authorized error, indicating that part of the sending network is on a blocklist (or blacklist).
Microsoft, perceive your email. If any portion of the shared IP's traffic triggers spam filters, the entire IP range can be flagged, leading to delivery failures for all legitimate senders. You might experience this as emails being filtered directly to the spam folder or, worse, completely blocked with hard bounces. This is a common challenge for those using Klaviyo shared IPs having Gmail deliverability issues too.
Salesforce) to initiate the process with ISPs.
Microsoft Postmaster tools.
Microsoft. Low deliverability and open rates are often symptoms of poor list hygiene.
Microsoft. If you find yourself repeatedly dealing with blocked bounces and struggling to get your messages delivered, it might be a sign that you need to grow out of a shared IP and consider a dedicated IP address. 
Microsoft domains, whether on shared or dedicated IPs, lies in a proactive approach to sender reputation. Continuously monitor your bounce rates, spam complaint rates, and inbox placement. Regularly review your email content and sending practices to ensure they align with best practices. For ongoing insights, remember to utilize why your emails are not delivering to Microsoft inboxes.
