How to troubleshoot Microsoft Outlook email block and irregular email volume?
Michael Ko
Co-founder & CEO, Suped
Published 5 Jul 2025
Updated 16 Aug 2025
9 min read
Dealing with Microsoft Outlook email blocks and irregular email volume can be a frustrating experience. It is particularly challenging when you encounter the generic smtp;550 5.7.1 error message, often accompanied by a S3150 code, indicating your IP is on a Microsoft blocklist. What makes it even more perplexing is when Microsoft's initial response to your mitigation request states they were unable to identify anything on our side that would prevent your mail from reaching Outlook.com customers.
This scenario, where your Smart Network Data Services (SNDS) portal shows a green status but emails are still bouncing, requires a methodical approach. It is not always about a clear, persistent blocklist entry but often relates to reputation filters and traffic patterns that trigger their internal systems.
Seasonal businesses, for instance, frequently face this issue due to significant fluctuations in their email sending volume. A sudden surge in email traffic, even legitimate, can be flagged as suspicious, mimicking compromised accounts or servers. This guide will walk you through the steps to troubleshoot these elusive blocks and manage irregular email volume effectively.
Understanding Outlook's blocking mechanisms
Microsoft's email infrastructure, including Microsoft 365 and Outlook.com, employs sophisticated spam and malware filtering that goes beyond simple blocklist checks. They use a combination of IP reputation, domain reputation, content analysis, and behavioral patterns to determine whether an email should be delivered to the inbox, junk folder, or blocked entirely. Irregular mail traffic (or abnormal email volume) is a significant red flag in their system.
Their systems are designed to protect users from spam and phishing, and a sudden, uncharacteristic spike in sending volume from an IP or domain, even if legitimate, can trigger automated defenses. This is because such changes can indicate a compromised account, a botnet, or a malicious actor attempting to send large volumes of unwanted emails. Thus, even if your IP isn't on a public blacklist or blocklist, Outlook's internal filters might still flag your sending due to these behavioral anomalies.
The SNDS portal provides insights into your sending reputation and spam complaint rates. However, as experienced email administrators often observe, a green SNDS status doesn't always guarantee deliverability. This discrepancy is what makes troubleshooting these specific blocks so challenging. The block may be a temporary, dynamic measure by Microsoft's SmartScreen filters, which are more sensitive to sending patterns than static blocklists.
A key piece of advice is to meticulously check all aspects of your email program, including SPF, DKIM, and DMARC records, and ensure your sending infrastructure is secure. Even if you're not on a formal blocklist, poor authentication can lead to deliverability issues.
Gathering evidence and preparing your case
When facing an Outlook email block, especially with an irregular volume flag, the first step is to gather all relevant data. Microsoft's initial response may be unspecific, but your detailed information is crucial for them to investigate further. You need to provide concrete evidence of the block and the context of your sending.
Begin by collecting complete bounce messages, including the exact error codes (like S3150) and any accompanying diagnostic information. Log data showing the volume of emails sent to Outlook.com addresses before, during, and after the incident is also vital. This includes daily or hourly sending rates.
Also, provide as much detail as possible about your client or sending entity. If your email volume is seasonal, clearly explain the typical sending patterns and why the current volume is considered normal for your operations (e.g., a ski resort sending more emails in winter). This context helps Microsoft understand that the irregular mail traffic they detect is benign.
Before contacting support, conduct an internal audit to ensure no changes on your end could have triggered the block. This includes recent email template changes or updates to your sending infrastructure. Knowing this beforehand allows you to address any self-inflicted issues and present a cleaner case to Microsoft.
Information to provide
Detailed bounce messages: Include full text, error codes (e.g., S3150), and time stamps.
Sending logs: Show daily/hourly volume to Outlook.com, successful vs. failed deliveries.
Context of sending: Explain seasonal or expected volume changes.
Internal checks before submitting
Recent changes: Verify if any template or infrastructure updates coincided with the block.
Authentication records: Confirm SPF, DKIM, and DMARC are correctly configured.
List hygiene: Ensure your mailing lists are clean and verified.
Engaging with Microsoft support
Engaging with Microsoft support for email deliverability issues can be a test of patience. Their initial responses are often automated or templated, typically stating they don't see an issue. This does not mean the problem isn't real or that they haven't identified anything, but rather that your request has not yet reached the appropriate level of investigation.
The key is persistence and providing additional detail with each reply. Treat their initial, unhelpful response as an open door to plead your case further. Reply to their email with the detailed bounce messages, sending logs, and the contextual information about your email volume. Politely ask for an escalation of your ticket to a higher tier of support. The more evidence you provide and the more you articulate your problem, the more likely you are to get a human agent to actively investigate.
Be prepared for a few rounds of communication. Even when a human responds, their communication might be limited to legal-approved templates. The goal is to keep the conversation going until they acknowledge the block and provide a solution. Many times, a mitigation response will come saying, We have implemented mitigation for your IP, though the fix might already be in effect by the time you receive the email. This process tests your patience, but it is often necessary to resolve Microsoft email blocking issues.
Remember to ask about SmartScreen filters if the issue persists, as these often play a role in dynamic blocking. Understanding why Microsoft is sending your emails to spam can sometimes lead to uncovering issues beyond simple blocklists.
Mitigating irregular volume and preventing future blocks
Once Microsoft indicates mitigation has been applied, it's not simply a switch that is flipped. The process can take 24-48 hours to replicate across their entire system. Moreover, if your issue stemmed from irregular email volume, proactive measures are essential to prevent future blocks. One effective strategy is to flatten your mail volumes. Instead of sending large blasts all at once, distribute your sending over several hours.
This approach, often called IP warming (even a mini version), helps rebuild or maintain your sending reputation gradually. Microsoft and other mailbox providers often have hourly and daily sending limitations tied to your perceived reputation. Sending slower, especially after a block or during a period of high volume, can significantly improve deliverability. You can learn more about this by reading technical solutions from top performing senders.
Another critical step is to rigorously maintain list hygiene. Regularly remove inactive subscribers and unengaged contacts. This reduces bounces and spam complaints, both of which negatively impact your sender reputation. While challenging for seasonal senders who might not have year-round engagement, it is still a best practice to purge very old, unengaged contacts.
Finally, consider the benefits of using dedicated IP addresses versus shared IPs. While shared IPs can be cost-effective, their reputation is influenced by other senders. Dedicated IPs offer more control over your sending reputation, making it easier to troubleshoot and mitigate issues like irregular volume blocks, as your sending patterns are the sole determinant of your IP's standing.
Before mitigation
Emails are bouncing with S3150 error.
SNDS portal might show a green status, which is confusing.
Initial Microsoft support replies are generic or unhelpful.
High volumes sent in short bursts.
After mitigation
Microsoft confirms mitigation implemented.
Implement mini IP warming to ramp up volume gradually.
Distribute sending volume over longer periods (e.g., 5 hours instead of 1).
Continue to maintain strict list hygiene.
Moving forward with confidence
Navigating email deliverability issues, particularly with a major mailbox provider like Outlook, requires a blend of technical understanding, strategic communication, and unwavering patience. While the process can be challenging, especially when facing vague error messages or initial unhelpful responses, a persistent and data-driven approach often yields positive results. The key is to remember that the system is designed to protect users, and providing context for your irregular volume can bridge the gap between automated detection and human understanding.
By meticulously collecting bounce logs, explaining your legitimate sending patterns, and persistently engaging Microsoft support for escalation, you increase your chances of successful mitigation. Post-mitigation, adopt strategies like volume flattening and consistent list hygiene to rebuild and maintain your sender reputation. Opting for dedicated IPs can also offer greater control and predictability over your deliverability. Ultimately, consistent best practices and proactive monitoring are your best defense against future blocks and irregular email volume flags.
Views from the trenches
Best practices
Provide full bounce messages and detailed sending logs for specific incidents.
Clearly explain the normal email volume patterns for your business, especially if seasonal.
Persist with Microsoft support, providing more evidence and requesting escalation.
Implement a mini IP warming schedule after mitigation to gradually increase volume.
Actively manage your mailing lists, removing unengaged contacts regularly.
Common pitfalls
Accepting the initial generic response from Microsoft support without further pushback.
Failing to gather comprehensive data (bounce messages, sending logs) before contacting support.
Ignoring internal checks for recent changes that might have triggered the block.
Sending a sudden, large volume of emails after a period of low activity.
Assuming a green SNDS status means there are no deliverability issues.
Expert tips
If possible, 'flatten' high mail volumes by distributing them over several hours or days.
Consider using dedicated IP addresses for more control over your sending reputation.
Be prepared for multiple rounds of communication with Microsoft support, as responses can be templated.
Even when mitigation is confirmed, allow 24-48 hours for the changes to fully propagate.
Look for content issues in your emails that might trigger spam filters, such as excessive links or suspicious keywords.
Marketer view
Marketer from Email Geeks says they often get vague responses from Microsoft, stating no issue, even when their IP is clearly blocked. They have to persist and show logs to get a proper investigation.
2022-01-14 - Email Geeks
Expert view
Expert from Email Geeks says to reply with bounce messages and details about mailing patterns, and to continuously ask for escalation to get Microsoft to dig deeper into SmartScreen issues.
