How do subdomains affect root domain reputation and how can I fix Microsoft O365 Outlook SCL:5 spam filtering issues?
Michael Ko
Co-founder & CEO, Suped
Published 8 Jun 2025
Updated 17 Aug 2025
8 min read
Dealing with email deliverability issues, especially when messages consistently land in spam folders, can be incredibly frustrating. I've encountered many scenarios where domains with seemingly perfect authentication and strong reputations with one major provider, like Google Postmaster Tools, still struggle to reach inboxes at others, most notably Microsoft 365 Outlook.
A common symptom of this is receiving a Spam Confidence Level (SCL) score of 5 for messages, which often results in them being junked. This happens even when your SPF, DKIM, and DMARC records are correctly configured, and you're sending marketing or transactional emails from subdomains.
The challenge lies in understanding how different mail providers assess reputation and the subtle ways subdomain activities can influence your primary domain's standing. Let's delve into these complexities and explore practical solutions for improving your deliverability.
How subdomains affect root domain reputation
One of the most frequent questions I encounter is whether subdomain reputation affects the root domain. The answer is yes, it typically does. While subdomains offer a layer of separation, allowing you to segment your email sending for different purposes, they are not entirely independent. A strong negative reputation on a subdomain can indeed impact the overall health of your root domain, particularly with major mailbox providers.
Many email filters employ algorithms that evaluate reputation at both the subdomain and root domain levels. This means if you're engaging in high-volume or potentially riskier sending activities, like cold outreach, on a specific subdomain, it's wise to acknowledge that any resulting spam complaints or negative engagement could trickle up. This cross-contamination means that issues like negative email engagement on a subdomain can affect your primary domain's standing.
Conversely, maintaining a good reputation on your subdomains can also contribute positively to the root domain's overall health. However, if your root domain is already suffering from a poor reputation, launching new subdomains won't instantly fix the problem. They might inherit some of the existing negativity, making it a prolonged process to recover your domain reputation.
Positive subdomain use
Purpose-built sending: Utilize subdomains to separate email types, such as marketing, transactional, and personal communications. This isolation limits the impact of one stream on another.
Reputation segmentation: A good reputation on a marketing subdomain can still enhance overall domain trust, but it may take time to offset a poor root domain reputation.
Scalability: Allows for different email service providers (ESPs) or sending configurations per subdomain, enabling tailored deliverability strategies.
Blacklisting risk: If a subdomain gets blocklisted, it can increase the likelihood of the parent domain (or other subdomains) also being placed on a blacklist (blocklist), leading to widespread delivery failures.
Conflicting data: Inconsistent sending quality across subdomains can make it harder for mailbox providers to accurately assess your overall sender reputation, sometimes leading to more cautious filtering.
Understanding SCL:5 in Microsoft 365
When your emails sent to Outlook or Office 365 mailboxes receive an SCL:5 rating, it means Microsoft's content filtering system has assessed the message as potentially spam. While SCL values can range from 0 (safest) to 9 (most likely spam), an SCL:5 specifically indicates that the message is considered 'normal' but still carries enough characteristics to be routed to the junk folder by default in many configurations.
It's important to understand that Microsoft's email filtering mechanisms operate independently from others. Even if Google Postmaster Tools reports a high reputation for your domain, it only reflects deliverability to Gmail addresses. Other providers maintain their own proprietary reputation systems, which analyze various factors including IP address reputation, domain reputation, content, sender behavior, and user engagement (e.g., how often recipients mark your emails as junk).
An SCL:5 score despite proper SPF, DKIM, and DMARC indicates that your authentication is likely in order, but other factors are flagging your mail. This often points towards a content issue, sender behavior pattern, or a general domain/IP reputation problem specific to Microsoft's assessment. My experience suggests that even personal emails can be affected if the overall domain reputation is low due to, for instance, a history of cold outreach or poor list hygiene.
SCL 5, 6: Potentially spam, sent to the junk email folder by default.
SCL 7, 8, 9: High confidence spam, quarantined or rejected.
Diagnosing Outlook deliverability issues
Diagnosing why your emails receive an SCL:5 score requires a systematic approach. One of the first steps is to capture and analyze the full email headers from a message that landed in junk. This will provide valuable insights into Microsoft's specific filtering decisions. Look for the X-Forefront-Antispam-Report header, which contains the SCL score and other diagnostic information.
I've often seen scenarios where basic authentication (SPF, DKIM, DMARC) checks out, but the emails still hit junk. This indicates that the problem is not a simple authentication failure, but rather a deeper reputation or content issue. Trying different SMTP services might yield similar results if the underlying domain reputation is the primary culprit, as most services ultimately share IP pools or are judged based on the sender domain.
Third-party inbox placement tools can be useful for quickly checking where your emails land across various providers, including Outlook. However, they typically don't provide the granular detail of the SCL score or the specific reasons for junking within Microsoft's environment. For more detailed analysis, you need to rely on the email headers themselves or direct feedback from Microsoft.
Pay close attention to user complaints. If your personal emails are also going to junk, it signifies a broader domain reputation problem, not just an issue with bulk sending. This could be due to factors like past cold outreach, poor list quality, or simply a lack of positive engagement signals from your recipients.
Issue
Indicators
Common causes
High SCL score (SCL:5+)
Emails consistently land in junk/spam folders.
Poor sending reputation with Microsoft, content issues, low recipient engagement.
Blocked messages
Emails are rejected entirely, often with bounce-backs.
IP or domain is on a blocklist, severe spamming history.
SPF/DKIM/DMARC failures
Authentication checks in email headers fail or are missing.
Incorrect DNS records, misconfigured sending services.
Strategies for improving Microsoft 365 Outlook deliverability
Fixing Microsoft O365 Outlook SCL:5 spam filtering issues often requires persistence and a clear understanding of how to communicate with Microsoft Support. One of the most effective methods I've found involves using very specific language when submitting support tickets to their sender support team. While they may initially respond with boilerplate text focusing on IP reputation, it's crucial to push for a domain reputation reset and an escalation.
I've learned that you often need to explicitly use terms like "domain reputation reset" and "escalation" in your replies. This specific phrasing seems to act as a "shibboleth", or a keyword that signals to their internal processes that your request needs different handling. You may need to repeat this cycle a few times, replying to their generic responses with these exact terms, until they escalate your case or reset your domain's standing. Keep pressing until they either stop responding (indicating they've done all they can) or confirm the reputation reset.
Beyond direct engagement with Microsoft, it's vital to address the underlying causes of poor reputation. This means continuously adhering to best practices: maintaining clean subscriber lists, avoiding cold outreach from your primary sending domains (or even subdomains if they affect the root), and ensuring consistent positive engagement from your recipients (encouraging them to move emails out of junk). Configuring anti-spam policies correctly within Office 365, including setting up allow lists for trusted senders, can also help. I also strongly advise against using email warm-up services as they can often do more harm than good, providing only temporary relief before potentially worsening your deliverability in the long run.
Engaging Microsoft support effectively
When contacting Microsoft's sender support for SCL:5 issues, remember these key points:
Be specific: Clearly state the problem is domain reputation-related, not IP, and that emails are landing in junk (SCL:5).
Use keywords: In your replies to their boilerplate, explicitly request a "domain reputation reset" and an "escalation."
Persist: It may take several rounds of communication to get your issue addressed by the appropriate team.
Views from the trenches
Best practices
Maintain separate subdomains for different email types, like marketing and transactional.
Consistently monitor your domain and IP reputation with all major mailbox providers, not just one.
Ensure all your email authentication records (SPF, DKIM, DMARC) are correctly configured.
Actively encourage recipients to move your emails from junk to inbox.
Regularly clean your email lists to remove inactive or invalid addresses.
Common pitfalls
Using cold outreach on your primary sending domains or closely related subdomains.
Ignoring SCL:5 scores, as they often lead to worse deliverability issues over time.
Relying solely on Google Postmaster Tools for your overall domain reputation.
Using third-party email warm-up services, which can backfire and harm reputation.
Failing to be persistent and clear when communicating with mailbox provider support.
Expert tips
Leverage the specific keywords like 'domain reputation reset' and 'escalation' when contacting Microsoft support.
Analyze full email headers, especially the 'X-Forefront-Antispam-Report' header, for detailed diagnostic information.
Understand that mailbox providers (like Microsoft) evaluate reputation independently of each other.
Implement DMARC policies at p=quarantine or p=reject to reinforce domain authenticity.
For personal emails landing in spam, advise recipients to move them to the inbox to build positive engagement signals.
Expert view
Expert from Email Geeks says cold email can significantly harm domain reputation, especially if the sending volume is aggressive. Google Postmaster Tools only reflects reputation for Gmail addresses and does not indicate reputation with business domains or other providers like Outlook or Microsoft 365.
2024-11-20 - Email Geeks
Expert view
Expert from Email Geeks says if emails are consistently landing in spam with an SCL:5 score, the issue is more likely related to domain reputation rather than IP reputation, especially when sending from large shared IP blocks like Google's. This is because each organization maintains its own reputation measurements.
2024-11-20 - Email Geeks
Mastering deliverability
Navigating email deliverability, especially with the nuances of Microsoft 365 Outlook filtering, can be a complex endeavor. While subdomains offer a strategic way to manage different sending reputations, it's clear they don't completely isolate the primary domain from negative impacts. Addressing SCL:5 issues requires a multi-pronged approach, combining diligent analysis of email headers and sender behavior with targeted, persistent communication with mailbox providers like Microsoft.
Microsoft 365 Outlook.
Outlook or 
Office 365 mailboxes receive an SCL:5 rating, it means Microsoft's content filtering system has assessed the message as potentially spam. While SCL values can range from 0 (safest) to 9 (most likely spam), an SCL:5 specifically indicates that the message is considered 'normal' but still carries enough characteristics to be routed to the junk folder by default in many configurations.
