What are Microsoft SCL and BCL ratings and how do they affect email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 23 Jun 2025
Updated 19 Aug 2025
7 min read
Email deliverability to Microsoft inboxes is a complex dance, and two key metrics often dictate whether your emails land in the inbox or the junk folder: Spam Confidence Level (SCL) and Bulk Complaint Level (BCL). These aren't just arbitrary numbers, they're critical indicators of how Microsoft's filtering systems perceive your sending reputation and email content.
Understanding these scores is paramount for anyone sending emails, whether transactional or marketing, as they directly impact your inbox placement. A low SCL or BCL can mean your legitimate emails are blocked or diverted, leading to missed opportunities and frustrated recipients. I've often seen clients with perfect deliverability elsewhere struggle significantly with Microsoft destinations, a problem frequently rooted in these scores.
Understanding Spam Confidence Level (SCL)
The Spam Confidence Level (SCL) is a numerical rating Microsoft Exchange Online Protection (EOP) assigns to incoming emails. It indicates the likelihood of a message being spam. The score ranges from -1 to 9, where a higher number signifies a greater probability of the email being junk. For example, an SCL of -1 suggests the email is from a safe sender and bypasses most spam filtering, while an SCL of 9 almost certainly means the email will be marked as spam.
Several factors contribute to an email's SCL. These include the sender's IP address and domain reputation, the content of the email itself, the use of certain keywords or phrases, and whether the message passes email authentication checks like SPF, DKIM, and DMARC. Even the formatting of your email can influence its SCL, with poorly formatted or suspicious-looking content increasing the score.
Example SCL HeaderHTTP
X-Forefront-Antispam-Report: ... SCL:5 ...
A high SCL can have severe consequences for your deliverability. Emails with an SCL of 5 or higher are typically routed to the junk mail folder by default in Microsoft 365. In some cases, very high SCLs can even lead to emails being quarantined or rejected outright. This means your recipients might never see your message, impacting your communication effectiveness and potentially your business operations. Understanding how to improve your SCL on Outlook is essential.
Decoding Bulk Complaint Level (BCL)
The Bulk Complaint Level (BCL) is another critical metric assigned by Microsoft to incoming emails, specifically those identified as bulk mail. Unlike SCL, which focuses on spam likelihood, BCL assesses how likely a bulk sender is to generate complaints. A higher BCL indicates that a sender's messages tend to result in more user complaints, even if the emails aren't strictly spam.
The definition of "bulk" can be nuanced. While it often refers to marketing or promotional emails, Microsoft broadly defines bulk mail based on characteristics like the number of recipients, the sending pattern, and the content type. A message being identified as bulk isn't inherently bad, but a high BCL associated with it signals issues. According to Microsoft's documentation, a BCL of 5, for instance, means the message is from a bulk sender that generates a mixed number of complaints.
High BCL scores are typically triggered by user behavior, such as recipients marking emails as junk, even if they originally opted in. This is where list hygiene and managing subscriber expectations become crucial. Even if your emails are technically compliant, if they consistently lead to complaints, your BCL will suffer, and your emails will likely end up in the junk folder. This is why addressing issues like high BCL scores is vital for sustained deliverability.
The interplay of SCL and BCL on deliverability
While SCL and BCL are distinct, they are deeply intertwined and both contribute to Microsoft's overall assessment of your email's deliverability. SCL primarily gauges whether a message looks like spam, taking into account factors like content analysis, sender reputation, and authentication. BCL, on the other hand, specifically measures the complaint rate associated with bulk mail from a particular sender.
Impact: Determines if email goes to inbox, junk, or is rejected based on perceived spamminess.
Bulk Complaint Level (BCL)
Focus: Tendency of bulk mail from a sender to generate complaints.
Influencers: User complaints (this is the primary factor), list quality, engagement metrics.
Impact: Affects deliverability for bulk senders, often leading to junk folder placement.
It's possible to have a low SCL (meaning the content isn't very spammy) but a high BCL (meaning many people complain about your bulk mail). This scenario often points to list hygiene issues, disengaged subscribers, or sending content that, while not explicitly spam, is unwelcome to recipients. Conversely, a good BCL but high SCL might indicate issues with email content, authentication, or a shared IP on a blocklist (or blacklist). Managing both effectively is key to improving email deliverability with Microsoft.
Strategies for improving your SCL and BCL
Improving your SCL and BCL ratings requires a holistic approach to email sending best practices. For SCL, focus on your sender reputation. Ensure your IP and domain are not on any major blocklists (or blacklists) and that your email authentication records (SPF, DKIM, DMARC) are correctly configured. A properly implemented DMARC policy, for example, can significantly boost your perceived trustworthiness. Microsoft's guidelines on SCL provide further detail on how these mechanisms are evaluated.
Best practices for a good SCL
Content Quality: Avoid spammy keywords, excessive exclamation marks, and suspicious formatting. Keep your content clear and concise.
Authentication: Implement SPF, DKIM, and DMARC correctly. These validate your sending domain and build trust.
Sender Reputation: Maintain a good reputation for your sending IP and domain. Avoid any activities that could lead to blocklist (or blacklist) listings.
For BCL, the focus shifts heavily to your audience and how they interact with your emails. Even with perfect authentication, a high complaint rate will inevitably lead to a high BCL. Regularly clean your email lists to remove inactive or disengaged subscribers. Implement double opt-in to ensure subscribers genuinely want your emails. Provide clear and easy unsubscribe options, as this is preferable to a spam complaint. Monitoring user engagement metrics and actively responding to feedback can help keep your BCL in check. To avoid your emails being marked as junk mail, proactive list management is key.
Navigating Microsoft's inbox filters
A low SCL and BCL are crucial for ensuring your emails reach Microsoft inboxes. SCL gauges how spammy your email looks, considering content and technical setup, while BCL measures the complaint rate of your bulk emails. Both scores are dynamic, influenced by your sending practices and recipient engagement.
To maintain optimal deliverability, consistent effort is required. This means regularly monitoring your sender reputation, ensuring proper email authentication, crafting engaging and relevant content, and diligently managing your subscriber lists to minimize complaints. By focusing on these areas, you can significantly improve your Microsoft deliverability and ensure your messages consistently reach their intended audience.
Views from the trenches
Best practices
Always use a double opt-in process for new subscribers to ensure high engagement and reduce complaint rates.
Regularly monitor your sending IP and domain reputation to catch any issues early and prevent blocklist (or blacklist) listings.
Segment your audience and tailor content to ensure relevance, reducing the likelihood of emails being marked as unwanted.
Provide a clear, one-click unsubscribe link in all marketing emails to allow disengaged users to opt-out gracefully.
Common pitfalls
Ignoring consistent junk folder placement for Microsoft recipients, assuming it's an isolated issue.
Failing to implement strong email authentication (SPF, DKIM, DMARC) which significantly impacts SCL.
Sending emails to old or unengaged lists, leading to high complaint rates and a poor BCL.
Relying solely on SCL or BCL without understanding their interplay and collective impact on deliverability.
Expert tips
A mixed SCL and BCL scenario often points to different root causes, so analyze both scores individually.
If your SCL is good but BCL is high, focus on list quality and content relevance for bulk mail.
If your IP is on a shared range, a good domain reputation can sometimes offset a questionable IP, but not always.
Microsoft's filtering can be very sensitive to user feedback, so prioritize recipient engagement above all else.
Expert view
Expert from Email Geeks says that SCL evaluates how likely a message is to be spam, while BCL indicates the likelihood of bulk mail from a sender generating complaints. Both contribute to deliverability, but from different angles.
2023-10-23 - Email Geeks
Marketer view
Marketer from Email Geeks says that if an email has a low SCL (not spammy) but a high BCL (quite spammy as bulk mail), the root cause is likely user complaints about the content being sent, rather than just technical reputation.
Microsoft inboxes is a complex dance, and two key metrics often dictate whether your emails land in the inbox or the junk folder: Spam Confidence Level (SCL) and Bulk Complaint Level (BCL). These aren't just arbitrary numbers, they're critical indicators of how Microsoft's filtering systems perceive your sending reputation and email content.
