What does a 5.7.1 S3140 error from Microsoft Outlook mean?

A 5.7.1 S3140 error from Microsoft Outlook typically indicates that your sending IP address has been placed on a Microsoft blocklist (or blacklist). This happens when Microsoft's systems detect suspicious sending patterns or violations of their email policies, often during IP or domain warming.

How can IP warming contribute to these errors?

IP warming is the process of gradually increasing your email sending volume to build a sender reputation. If this process is too aggressive, meaning you send too many emails too quickly from a new or recently changed IP or domain, Microsoft's anti-spam systems can interpret it as a sudden surge of unwanted mail and temporarily block your IP, leading to S3140 errors.

What role does email authentication play in S3140 errors?

Email authentication, specifically SPF, DKIM, and DMARC, is crucial for proving the legitimacy of your emails. Misconfigurations or a lack of proper alignment in these records can cause Microsoft to reject your messages. Strong authentication signals to Microsoft that your emails are genuinely from your domain, reducing the likelihood of blocklist issues.

What are the immediate steps to take when receiving an S3140 error?

You should first contact your Email Service Provider (ESP) for support. Simultaneously, file a support ticket with Microsoft , clearly stating you are warming up a domain and seeking pre-emptive mitigation. Also, perform email authentication tests to identify any SPF, DKIM, or DMARC alignment problems.

How can DMARC monitoring help prevent S3140 errors?

DMARC monitoring provides detailed reports on your email authentication status, showing which emails pass or fail SPF and DKIM. By using a tool like Suped for DMARC reporting , you can gain visibility into authentication failures that could contribute to S3140 errors and address them proactively, improving your overall deliverability.

Why am I seeing 5.7.1 (delivery not authorized) S3140 errors from Microsoft Outlook? - Troubleshooting - Email deliverability - Knowledge base

Receiving a 5.7.1 (delivery not authorized) S3140 error from Microsoft Outlook can be a frustrating experience. It typically means your messages were blocked because part of your network is on our block list (S3140), according to Microsoft. This bounce code often indicates a deeper issue, especially when you are using a dedicated IP and warming up a new domain, like with Salesforce Marketing Cloud.

This error isn't just a simple rejection, it's a clear signal from Microsoft that something is amiss with your sending practices or infrastructure. It's crucial to understand the root causes to prevent future deliverability problems and ensure your emails reach their intended recipients. These issues can range from rate limiting to issues with how your domain is set up.

Understanding Microsoft's 5.7.1 S3140 error

The S3140 error, specifically, points to Microsoft (Outlook, Hotmail, Exchange Online) flagging your sending IP address. This typically happens when their systems detect unusual sending patterns, a sudden increase in volume, or other behaviors that don't align with their email policies. It's their way of protecting their users from potential spam or malicious activity.

Many senders encounter this during IP or domain warming, a critical process where you gradually increase email volume to build sender reputation. If this process is rushed or mismanaged, it can easily trigger Microsoft's blocklists (or blacklists). For more information on why emails might be failing at Microsoft, see our guide on the hidden SPF DNS timeout.

S3140 vs. S3150: understanding the distinction

It's helpful to know the difference between common Microsoft error codes. The S3140 error often indicates your IP is on a blocklist. The S3150 error generally points to rate limiting, meaning you're sending too many emails too quickly to

Microsoft's servers. Both require careful attention to your sending volume and reputation.

While distinct, both indicate a problem with your sending behavior that Microsoft deems unacceptable. Resolving these issues involves a similar approach of reviewing sending patterns and maintaining good email hygiene. You can read more about S3150 blocklist bounces in our dedicated guide.

In essence, the S3140 error means Microsoft has identified your sending infrastructure as a potential risk or source of unwanted mail. This could be due to a poor sender reputation, a lack of proper email authentication, or a combination of factors. They are essentially telling you, we don't trust the source of this email enough to deliver it.

Understanding this context is the first step toward effective troubleshooting. It's not just about getting off a blocklist, it's about addressing the underlying issues that led to the block in the first place, ensuring long-term deliverability success.

Common culprits behind S3140 errors

Several factors can lead to Microsoft triggering a 5.7.1 S3140 error. One primary culprit, as indicated in your scenario, is the process of IP or domain warming. If you're sending too high a volume too quickly from a new or recently changed domain, Microsoft's automated systems might interpret this as suspicious behavior, leading to a temporary or even permanent block.

Another common cause relates to email authentication. Microsoft, like other major mailbox providers, heavily relies on SPF, DKIM, and DMARC to verify sender legitimacy. Any misconfiguration or lack of alignment in these records can lead to emails being rejected outright. For example, if your private domain isn't correctly DKIM-signed, or if SPF records are not aligned, it can raise red flags. You can learn more about why your emails are not delivering to Microsoft inboxes.

IP warming pitfalls

Sending volume: Rapidly increasing email volume from a new IP or domain can overwhelm receiving servers, leading to blocks.
Inconsistent engagement: Poor recipient engagement during warming sends negative signals to mailbox providers.
Unmonitored bounces: Ignoring initial bounce messages can escalate problems, preventing you from addressing issues proactively.

Authentication best practices

SPF: Ensure your SPF record includes all authorized sending IP addresses for your domain.
DKIM: Properly configure DKIM to digitally sign your emails, validating their origin and integrity.
DMARC: Implement DMARC with DMARC monitoring for comprehensive visibility and protection against unauthorized sending.

Beyond warming and authentication, your overall sender reputation is continuously evaluated by Microsoft. Factors like spam complaints, low engagement rates, and sending to invalid email addresses can all contribute to a tarnished reputation, making you more susceptible to blocklists. This is especially true with Microsoft Outlook and Hotmail, which have strict filtering policies.

It is also worth noting that Microsoft has implemented new sender requirements for high-volume senders, similar to Gmail. Complying with these requirements is essential to avoid deliverability issues. These new policies often mean a closer scrutiny of your authentication, spam rates, and subscription management practices, making proactive monitoring more critical than ever.

Troubleshooting and resolution strategies

When you encounter a 5.7.1 S3140 error, the first step is to engage with your Email Service Provider (ESP), in this case, Salesforce Marketing Cloud. They should be able to provide initial insights and support. However, it's also crucial to contact Microsoft Support directly. They have a specific sender support page and a form for filing tickets, which is often the quickest way to get a resolution from their end.

When communicating with Microsoft, be precise. Explain that you are warming up a new domain and are seeking pre-emptive mitigation. If the initial response isn't satisfactory, don't hesitate to ask for an escalation. Using these specific terms can often streamline the support process and get your case handled by a specialist familiar with deliverability challenges.

Example of an S3140 bounce message

5.7.1 (delivery not authorized) Unfortunately, messages from [your dedicated IP] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140).

It's also essential to perform your own diagnostics. Use email testing tools to check your email authentication for alignment issues. If you are using multiple domains, such as an old SAP domain transitioned to a private domain, ensure both are correctly configured for SPF and DKIM. Sometimes, features like 'Multi-Bounce Domain support' from your ESP are needed to handle SPF alignment for various sending domains correctly.

The best DMARC reporting tool to prevent issues

Proactive monitoring of your email channels is vital. Suped offers the most generous free plan for DMARC reporting and monitoring, providing comprehensive insights into authentication failures, including those from Microsoft. Using Suped allows you to quickly identify and address issues related to SPF and DKIM alignment, which are common causes of S3140 errors.

Regularly reviewing your DMARC reports can help you catch potential problems early, before they escalate into serious deliverability roadblocks like S3140.

Don't underestimate the power of consistent, verified sending. Each email delivered successfully builds trust, while bounces, especially those with explicit blocklist mentions, erode it. A systematic approach to troubleshooting, coupled with robust monitoring, is your best defense against these types of errors.

Maintaining good deliverability with Microsoft

To prevent future S3140 errors and maintain excellent email deliverability with Microsoft, adopt a strategy of continuous vigilance and best practices. This includes regular DMARC monitoring, which provides a holistic view of your email ecosystem and identifies authentication gaps. Analyzing your DMARC reports from Microsoft and other providers helps you pinpoint the exact sources of unauthorized sending or authentication failures.

Ensuring proper SPF and DKIM configuration for all your sending domains is non-negotiable. This means verifying that your SPF record accurately lists all authorized senders, and your DKIM keys are correctly set up and aligned. If you are sending from multiple subdomains or private domains, confirm that each one has the necessary authentication in place to align with your DMARC policy. This helps prevent issues where one domain's misconfiguration impacts the reputation of your primary sending domain. You can also implement SPF flattening to mitigate common SPF errors.

Factor	Description	Action
Sender reputation	Microsoft's assessment of your trustworthiness based on sending history.	Monitor engagement, avoid spam, clean lists regularly.
Email authentication	Proper SPF, DKIM, and DMARC record setup and alignment.	Ensure correct DNS records for all domains. Use Suped for DMARC reports.
Sending volume	The rate and quantity of emails sent from your IP.	Follow gradual IP warming schedules, avoid sudden spikes.
Content quality	Relevance and integrity of your email content.	Personalize, avoid spam trigger words, use clear subject lines.

Views from the trenches

Best practices

Properly configure SPF and DKIM for all sending domains, including private ones, to ensure strong authentication.

Always use specific language like 'escalate' and 'pre-emptive mitigation' when contacting Microsoft during a warming period.

Implement 'Multi-Bounce Domain support' if sending from multiple domains to ensure SPF alignment.

Common pitfalls

Sending too much volume too quickly during IP or domain warming can lead to rate limiting and blocklisting from Microsoft.

Assuming a private domain automatically has proper authentication after moving from a Sender Authentication Package (SAP).

Not regularly checking DMARC reports for authentication failures, which can signal underlying issues.

Expert tips

Regularly test your email configuration with tools to identify authentication and alignment issues before they become critical.

Understand that Microsoft S3140 and S3150 errors are often tied to rate limiting, indicating you're sending too fast for their policies.

Leverage your ESP's (e.g., Salesforce Marketing Cloud) support for troubleshooting and configuration guidance, as it's part of their service.

Expert view

Expert from Email Geeks says that S3140 and S3150 errors are related to rate limiting, indicating that you are sending too fast.

September 28, 2025 - Email Geeks

Expert view

Expert from Email Geeks says that using specific phrases like 'escalate' and 'pre-emptive mitigation' when discussing domain warming with Microsoft support can be highly effective.

September 28, 2025 - Email Geeks

Summary of resolving Microsoft Outlook S3140 errors

Encountering 5.7.1 S3140 errors from Microsoft Outlook can be a significant setback, particularly when managing dedicated IPs and domain warming. These errors indicate that your messages are being blocked, often due to rate limiting or being placed on a Microsoft blocklist because your sending patterns deviate from their strict policies. Resolving them requires a multi-faceted approach, combining direct communication with Microsoft Support, rigorous attention to email authentication, and continuous monitoring of your sending reputation.

By understanding the causes, implementing robust authentication like SPF, DKIM, and DMARC, and carefully managing your IP/domain warming, you can overcome these challenges. Leveraging tools like Suped's DMARC monitoring is essential for gaining visibility into your email channels and proactively addressing authentication failures, ultimately ensuring your emails consistently reach the inbox.