Why are my emails marked with a high BCL score and landing in junk folders, and how can I fix it?
Matthew Whittaker
Co-founder & CTO, Suped
Published 27 Apr 2025
Updated 16 Aug 2025
8 min read
It's frustrating to see your carefully crafted emails consistently land in junk or spam folders, especially when you're doing everything right with your email authentication. Many senders, particularly those using Microsoft 365 or Outlook, encounter this issue, often linked to a high Bulk Complaint Level (BCL) score. This score is a key indicator Microsoft uses to assess whether an incoming email stream is legitimate or undesirable bulk mail.
A high BCL score means Outlook perceives your emails as more likely to generate complaints, leading to poor inbox placement. Understanding why this score gets assigned and how to mitigate it is crucial for maintaining good email deliverability. It is not just about technical setup, but also about content, recipient engagement, and sender practices.
This guide will walk you through the intricacies of BCL scores, what causes them to rise, and actionable steps you can take to lower your score and ensure your emails reach the inbox, not the junk folder.
Understanding BCL scores
The Bulk Complaint Level (BCL) score is a proprietary metric used by Microsoft's email filtering systems to determine the likelihood that an incoming bulk email will generate complaints. It ranges from 0 to 9, where 0 indicates very low complaint probability, and 9 indicates a very high probability. Essentially, it's Microsoft's assessment of how spammy (or unwanted) your bulk mail stream appears.
Unlike some other reputation metrics that focus on individual sender IP or domain reputation, BCL specifically targets bulk senders. Microsoft Defender for Office 365 uses BCL in conjunction with other signals, such as Spam Confidence Level (SCL), to make a final filtering decision. Even if your SPF, DKIM, and DMARC authentication records are perfectly set up, a high BCL can still direct your emails to the junk folder.
The precise algorithm for BCL is not publicly disclosed, but it's known to factor in various signals from both Microsoft's internal data and third-party sources. These signals typically include user complaints, spam trap hits, engagement metrics (or lack thereof), and content analysis. A key point is that an organization's Office 365 administrator can configure their own bulk email thresholds, meaning that a BCL of 4 might be fine for one recipient but land in junk for another.
Common causes of high BCL scores
Several factors can contribute to a rising BCL score, even if your email authentication is technically sound. These often boil down to how recipients interact with your emails and the overall quality of your sending practices. It's not always about outright malicious intent, but often about perceived unwanted email.
Recipient engagement: Low open rates, high delete-without-reading rates, and a significant number of users moving your emails from the inbox to the junk folder or marking them as spam can severely impact your BCL. Microsoft values positive engagement.
List hygiene: Sending to invalid email addresses, spam traps, or very old lists signals poor sender practices. This can lead to a quick increase in your BCL score or even get your domain or IP placed on a blocklist (or blacklist).
Content issues: While less about explicit spam words now, things like excessive imagery, poor HTML, broken links, or content that consistently triggers user complaints can contribute. This includes content that might be perceived as phishy or deceptive.
Sending volume spikes: Sudden increases in email volume from a previously low-volume sender can trigger bulk filters. This is why proper IP and domain warming is essential when scaling your email program.
It is also worth noting that using a shared IP (where other senders' reputations can impact yours) or having a poor overall sender reputation can indirectly lead to a higher BCL score.
Diagnosing and fixing high BCL scores
The first step to fixing a high BCL score is diagnosing it. You can check the BCL score by examining the email headers of a message that landed in junk. Look for the X-Microsoft-Antispam header.
Example of X-Microsoft-Antispam headertext
X-Microsoft-Antispam: BCL:4;
Once you've identified a high BCL score, here's how to tackle it:
Improve list quality: Regularly clean your email lists to remove inactive or invalid addresses. Implement double opt-in for new subscribers. This reduces hard bounces and prevents you from hitting spam traps, which significantly improves your sender reputation.
Optimize content: Ensure your email content is relevant, personalized, and provides clear value. Avoid overly promotional language, excessive capitalization, or suspicious links. A plain text version of your email can also help. Reviewing automated templated emails is a good starting point.
Warm up new sending infrastructure: If you're using a new IP address or domain, gradually increase your sending volume to build a positive reputation with ISPs. This phased approach helps prevent your emails from being flagged as suspicious bulk mail.
Monitor engagement metrics: Pay attention to your open rates, click-through rates, and complaint rates. High complaint rates or low engagement can signal to Microsoft that your emails are unwanted, leading to a higher BCL.
It is also crucial to ensure your email authentication, including SPF, DKIM, and DMARC, is correctly configured and aligned. While BCL is not directly tied to DMARC alignment, proper authentication strengthens your overall sender reputation and trustworthiness, which indirectly affects how Microsoft perceives your mail.
Long-term deliverability strategies
Beyond immediate fixes, building a robust long-term deliverability strategy is key to preventing future BCL issues. This involves continuous monitoring, proactive engagement management, and adherence to best practices.
Implement feedback loops: Sign up for Microsoft's Junk Mail Reporting Program (JMRP). This program, and similar ones from other ISPs, notifies you when recipients mark your emails as junk, allowing you to promptly remove those users from your list. This is vital for lowering complaint rates and maintaining a good sender reputation.
Segment your audience: Send highly targeted content to engaged segments of your audience. This increases relevance and reduces the likelihood of complaints, thus keeping your BCL score low. Tailoring your email content to specific interests helps ensure it's wanted by the recipient.
Monitor blocklists: Regularly check if your sending IP or domain is on any major blocklists (or blacklists). While not directly BCL-related, being on a major blocklist will almost guarantee poor inbox placement and contribute to a high BCL.
By proactively managing these aspects of your email program, you can significantly improve your BCL score and ensure your emails consistently reach the inboxes of your Microsoft 365 and Outlook recipients.
The path to better inbox placement
When your emails consistently land in junk or are marked with a high BCL score, it's a clear signal that Microsoft's filtering systems perceive your mail as unwanted bulk. Addressing this requires a holistic approach that goes beyond basic authentication. Focus on recipient engagement, meticulous list hygiene, gradual warming of new sending infrastructure, and continuous monitoring.
By implementing these strategies, you can significantly improve your BCL score, enhance your sender reputation, and ensure your emails consistently reach their intended destination: the inbox. Proactive email deliverability management is an ongoing process, but the effort pays off in better engagement and successful campaigns. Ultimately, your goal is to send emails that your recipients want to receive.
Views from the trenches
Best practices
Actively use engagement data to refine your sending strategy and remove inactive users to prevent complaints.
Regularly audit your email content for anything that might be perceived as spammy or low-value.
Implement a double opt-in process for all new subscribers to ensure high-quality, engaged leads.
Segment your email lists and tailor content to ensure maximum relevance for each recipient group.
Common pitfalls
Ignoring low open rates and high complaint rates, which directly impact your BCL score.
Failing to warm up new sending IPs or domains, leading to sudden spikes in volume and filtering issues.
Not cleaning your email lists regularly, which can lead to hitting spam traps and high bounce rates.
Relying solely on technical authentication (SPF/DKIM/DMARC) without addressing content or engagement issues.
Expert tips
BCL is primarily a Microsoft score related to bulk mail, so focus your troubleshooting efforts on Microsoft properties.
A low BCL score (e.g., 0) doesn't guarantee inbox delivery; other factors like content and engagement still matter.
If SPF alignment is failing with Amazon SES, configure a custom MAILFROM domain for the Return-Path.
Understand that Office 365 administrators can set their own BCL thresholds, affecting individual delivery.
Expert view
Expert from Email Geeks says the BCL header is unrelated to DMARC alignment and that for Amazon SES, you need to talk to them about custom domains in the return path.
2021-07-15 - Email Geeks
Expert view
Expert from Email Geeks says BCL is a score assigned by Microsoft from 0 to 9, indicating confidence in mail being bulk. Office 365 customers set their own thresholds for when mail is considered spam, leading to junking, quarantining, or discarding.
Microsoft 365 or Outlook, encounter this issue, often linked to a high Bulk Complaint Level (BCL) score. This score is a key indicator Microsoft uses to assess whether an incoming email stream is legitimate or undesirable bulk mail.
Outlook perceives your emails as more likely to generate complaints, leading to poor inbox placement. Understanding why this score gets assigned and how to mitigate it is crucial for maintaining good email deliverability. It is not just about technical setup, but also about content, recipient engagement, and sender practices.
Microsoft's email filtering systems to determine the likelihood that an incoming bulk email will generate complaints. It ranges from 0 to 9, where 0 indicates very low complaint probability, and 9 indicates a very high probability. Essentially, it's Microsoft's assessment of how spammy (or unwanted) your bulk mail stream appears.
ISPs. This phased approach helps prevent your emails from being flagged as suspicious bulk mail.
Microsoft's Junk Mail Reporting Program (JMRP). This program, and similar ones from other ISPs, notifies you when recipients mark your emails as junk, allowing you to promptly remove those users from your list. This is vital for lowering complaint rates and maintaining a good sender reputation.
