Summary
The Bulk Complaint Level (BCL) score, particularly a BCL 6, within Outlook email deliverability, primarily reflects a high likelihood of bulk mail generating complaints from recipients. This score is predominantly driven by the volume and consistency of user complaints, signaling to Microsoft that the sender's mail is unwanted. Alongside direct user feedback, factors such as the sender's IP reputation, domain reputation, and encounters with spam traps also contribute to this internal Microsoft metric. A BCL 6 indicates a poor sender reputation, resulting in a higher chance of emails being filtered into the junk folder.
Key findings
- Recipient Complaints are Primary: The most significant factor influencing a BCL 6 score in Outlook is a high volume and consistent pattern of recipients marking emails as spam or junk. A BCL 6 explicitly indicates a high likelihood of sending bulk mail that generates complaints.
- IP and Domain Reputation: While recipient complaints are paramount, a BCL score, including BCL 6, is also influenced by broader sender reputation factors such as the sending IP's reputation and the sender's domain reputation. Some sources suggest IP reputation can be a nearly complete basis for the BCL.
- Spam Traps and Blacklistings: Beyond direct user complaints, other indicators of poor sender reputation like hits on spam traps and existing blacklistings can contribute to an elevated BCL score, signaling a high probability of the sender being a source of spam.
- Internal Microsoft Metric: The BCL score is an internal Microsoft reputation metric. It assesses how likely an email is to generate complaints and directly impacts an email's deliverability to Outlook inboxes, often leading to messages being filtered into junk folders.
Key considerations
- Complaint Reduction Strategies: To mitigate a BCL 6 score, email senders should intensely focus on reducing recipient complaints. This involves strategies like sending only to engaged subscribers, providing clear unsubscribe options, and ensuring content relevance.
- Maintain List Hygiene: Proactive list hygiene, which includes regularly cleaning inactive subscribers and removing unengaged contacts, is crucial to avoid spam traps and reduce complaint rates, thus positively influencing the BCL score.
- Monitor Sender Reputation: A BCL 6 indicates a poor sender reputation with Microsoft. Senders should continuously monitor their sending practices, IP and domain health, and engagement metrics to prevent or recover from such a score.
What email marketers say
13 marketer opinions
Outlook's Bulk Complaint Level (BCL) score, particularly a BCL 6, signals severe deliverability issues, primarily stemming from a high volume of recipient spam complaints. This metric indicates to Microsoft that the sender's bulk email is largely unwanted by users, significantly impacting inbox placement. While user complaints are the dominant influence, the BCL score also incorporates aspects of the sender's IP and domain reputation, as well as encounters with spam traps.
Key opinions
- High Complaint Rates: A BCL 6 score is overwhelmingly influenced by a high volume and sustained pattern of recipients reporting emails as spam or junk. This consistent negative feedback is Microsoft's primary indicator that bulk mail is unwanted.
- IP and Domain Reputation: Beyond direct recipient feedback, the BCL score, including a 6, is significantly influenced by the reputation of the sending IP and the associated domain. Some insights suggest IP reputation can be a nearly complete determinant of this score.
- Spam Traps and Engagement: The BCL score is also affected by hits on spam traps, which indicate poor list quality. Community discussions suggest that long-term sender history and consistently low engagement can contribute to a negative reputation that makes recovering from a high BCL score more difficult.
Key considerations
- Reduce Complaints: The most crucial action to prevent or mitigate a BCL 6 score is to drastically reduce the volume of recipient complaints. This involves strong opt-in practices, prominent unsubscribe links, and delivering content that consistently meets subscriber expectations.
- List Quality and Engagement: Maintaining a clean, engaged email list is vital. Regularly pruning inactive subscribers helps avoid spam traps and reduces the likelihood of complaints, contributing positively to your BCL score.
- Proactive Reputation Management: Given that a BCL 6 indicates a poor sender reputation with Microsoft, it's essential for senders to proactively manage their IP and domain reputation, address any blacklistings, and ensure consistent adherence to email best practices to rebuild trust.
Marketer view
Marketer from Email Geeks explains that BCL is believed to be nearly completely based on IP reputation.
29 Aug 2023 - Email Geeks
Marketer view
Marketer from Email Geeks explains that X-Microsoft-Antispam is believed to be largely based on IP reputation, but a sourced definition for X-Exchange-Antispam-Report-CFA-Test is not available.
1 May 2024 - Email Geeks
What the experts say
1 expert opinions
Outlook's internal Bulk Complaint Level (BCL) score, particularly a BCL 6, indicates a high likelihood of an IP address being a source of spam. This score is directly influenced by critical reputation signals, including the volume of spam complaints, occurrences of spam trap hits, and presence on blacklists. To mitigate a high BCL, senders must focus on improving mail quality, practicing rigorous list hygiene, and enhancing recipient engagement.
Key opinions
- Spam Probability Indicator: A BCL 6 score explicitly indicates a high probability that the sending IP address is a source of spam, as assessed by Microsoft's internal reputation system.
- Core Reputation Signals: The primary factors influencing a BCL 6 score are common negative reputation signals, specifically spam complaints, hits on spam traps, and instances of blacklisting.
- Microsoft's Internal Metric: The Bulk Complaint Level (BCL) is an internal Microsoft reputation metric designed to assess the likelihood of an IP address sending unwanted or spam-like email.
Key considerations
- Improve Mail Quality: Senders should prioritize enhancing the overall quality and relevance of their email content to reduce the likelihood of recipient complaints and improve reputation signals.
- Optimize List Hygiene: Regular and thorough cleaning of email lists is essential to remove inactive or invalid addresses, thereby avoiding spam traps and positively influencing the BCL score.
- Boost Recipient Engagement: Focusing on strategies to increase positive recipient engagement and minimize negative feedback is crucial for rebuilding sender reputation and lowering a high BCL score.
Expert view
Expert from Word to the Wise explains that the Blocklist Confidence Level (BCL) is an internal Microsoft reputation metric that assesses the likelihood of an IP address sending spam. A BCL 6 indicates a high probability of the sender being a source of spam. The key factors influencing this score are common reputation signals such as spam complaints, spam trap hits, and blacklistings. To lower a high BCL, senders should focus on improving mail quality, list hygiene, and recipient engagement.
22 Oct 2022 - Word to the Wise
What the documentation says
3 technical articles
Outlook's Bulk Complaint Level (BCL) score, particularly a BCL 6, is profoundly shaped by the volume of complaints a sender's bulk email generates from recipients. This metric, which is internal to Microsoft, functions as a direct indicator of how likely messages are to be marked as unwanted. Factors such as the overall sender IP and domain reputation are also integrated into this score, with a consistent pattern of user complaints being a critical determinant leading to a BCL 6 and subsequent junk folder placement.
Key findings
- User Complaints Are Key: The most significant determinant of a BCL 6 score is the consistent volume of recipients marking bulk emails as spam or junk, indicating a high likelihood that the mail is unwanted.
- IP and Domain Reputation: Beyond direct complaints, the BCL score integrates broader sender reputation factors, including the health of the sending IP address and the associated domain's reputation.
- Microsoft's Internal System: The Bulk Complaint Level (BCL) is an internal metric used by Microsoft to assess the propensity of an email to generate complaints, directly impacting deliverability within Outlook.
Key considerations
- Prioritize Complaint Reduction: The most impactful action for senders is to drastically reduce recipient complaints by maintaining strong opt-in practices, providing prominent unsubscribe options, and ensuring content relevance to subscriber expectations.
- Maintain Robust Sender Health: Given that IP and domain reputation contribute to the BCL, senders must continuously monitor their sending infrastructure and adhere to best practices to maintain a positive sender health score with Microsoft.
- Focus on Subscriber Engagement: Cultivating a highly engaged subscriber base and regularly cleaning inactive contacts helps minimize negative feedback and avoids spam traps, which can indirectly influence the BCL score.
Technical article
Documentation from Microsoft Learn explains that sender reputation, which includes the Bulk Complaint Level (BCL), is influenced by factors like sender IP, domain reputation, and spam complaints from users. A BCL score of 6 indicates a high likelihood of sending bulk mail that generates complaints.
14 Aug 2022 - Microsoft Learn
Technical article
Documentation from Microsoft Learn clarifies that the Bulk Complaint Level (BCL) is an indicator of how likely an email is to generate complaints. While not explicitly listing all factors that cause a BCL6 score, it implies that consistent patterns of user complaints for bulk mail contribute significantly to this rating, leading to messages being marked as spam.
2 Jun 2025 - Microsoft Learn
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