What factors influence a BCL 6 score in Outlook email deliverability?

Key findings

• IP reputation: The sending IP address's reputation is widely considered a primary factor influencing the BCL score. A history of high complaints or undesirable sending behavior from an IP will negatively impact its BCL.
• Domain reputation: While IP reputation is often emphasized, the sending domain's reputation, including its history of complaints and authentication, also contributes to the BCL calculation. For more details on domain reputation, see our guide on what factors influence email deliverability.
• Complaint rates: A BCL of 6 directly indicates that the sender is generating a "mixed" number of complaints from Microsoft users, suggesting inconsistent email practices that lead to negative feedback.
• Multiple headers: Outlook emails may contain multiple BCL headers (e.g., X-Microsoft-Antispam and X-Exchange-Antispam-Report-CFA-Test), which sometimes have differing values, implying that different criteria or stages of evaluation might be at play.
• Content's indirect role: While BCL primarily reflects sender reputation and complaint volume, content can indirectly influence it by triggering complaints. Poorly targeted or spammy content is more likely to be reported, thereby increasing complaint rates and raising the BCL.

Key considerations

• Proactive reputation monitoring: Regularly monitor your IP and domain reputation to identify and address issues before they lead to a high BCL score. Tools and postmaster data can provide valuable insights.
• Complaint management: Implement feedback loops to quickly identify and remove subscribers who mark your emails as spam. This proactive approach helps reduce overall complaint rates, a direct factor in BCL scores.
• List hygiene: Maintain a clean, engaged subscriber list. Regularly remove inactive or unengaged subscribers who are more likely to mark your emails as junk, which can lower your BCL over time. Find out more about why emails end up in junk in Microsoft Outlook.
• Authentication: Ensure your emails are properly authenticated with SPF, DKIM, and DMARC. Strong authentication builds trust with ISPs, contributing positively to your sender reputation and indirectly to your BCL. Learn more about recovering domain reputation.

Key opinions

• IP reputation as primary: Many marketers believe that the BCL score is predominantly, if not entirely, based on the reputation of the sending IP address, especially for the X-Microsoft-Antispam header.
• Mixed complaint interpretation: Marketers interpret a BCL:6 score as an indication that their sender (IP/domain) is generating a "mixed number of complaints," suggesting inconsistency in recipient feedback.
• Header differentiation: Observations show that BCL scores can appear in different headers, such as X-Microsoft-Antispam and X-Exchange-Antispam-Report-CFA-Test, and sometimes differ, leading to speculation that distinct criteria (e.g., IP vs. content) might be evaluated for each.
• Sender definition ambiguity: There's often a question about what "sender" specifically refers to in the context of BCL: whether it's the IP, the return-path domain, or the From domain, with IP being the most common suspicion.

Key considerations

• Complaint reduction strategies: Marketers should prioritize strategies that minimize spam complaints, as this is the most direct way to influence BCL. This includes clear consent, relevant content, and easy unsubscribe options.
• Audience engagement: Focusing on sending emails to a highly engaged audience reduces the likelihood of complaints and positive signals to Outlook. Consider how managing inactive email subscribers can improve deliverability.
• Holistic deliverability view: While BCL is specific to Microsoft, marketers should maintain overall email deliverability best practices, as many factors are interconnected across ISPs. This includes regular deliverability best practices for Outlook and Gmail.
• Troubleshooting methodology: When facing a BCL 6, marketers should systematically review their sending behavior, list acquisition, and content to identify potential root causes for increased complaints. For further reading, explore why your emails are going to spam.

Key opinions

• IP reputation strongly influences: Experts widely believe that the BCL score, especially the one associated with the X-Microsoft-Antispam header, is heavily, if not largely, determined by the sending IP's reputation for generating complaints.
• Combined IP and domain factors: Some experts state that BCL can be based on a combination of both IP and domain reputation, indicating that Microsoft considers a broader set of sender attributes.
• Ambiguity in secondary headers: There's less definitive information available from sourced definitions for secondary BCL headers, such as X-Exchange-Antispam-Report-CFA-Test, suggesting they might involve other, less understood criteria.
• Complaint rate metric: A BCL of 6 is understood to mean the sender generates a "mixed number of complaints," reflecting a reputation that is neither excellent nor consistently poor.

Key considerations

• Comprehensive reputation management: Experts advise focusing on improving both IP and domain reputation. This involves maintaining low complaint rates, ensuring proper authentication (SPF, DKIM, DMARC), and adhering to best practices that signal trustworthiness to ISPs.
• Proactive monitoring: Beyond BCL, closely monitor other reputation metrics and DMARC reports to catch negative trends early. Learn more about understanding your email domain reputation.
• Address underlying issues: A BCL of 6 is a symptom. Experts recommend delving into the root causes of complaints, such as poor list quality, irrelevant content, or frequency issues. This can often involve examining internal blocklist listings.
• Engaged audience strategy: Cultivating an engaged subscriber base through opt-in processes and relevant content is crucial. Experts at Word to the Wise frequently highlight this principle.

Key findings

• Direct complaint correlation: Microsoft's documentation directly links the BCL score to the sender's history of generating complaints from users, indicating it's a direct reflection of negative recipient feedback.
• Sender reputation metric: BCL is presented as a sender reputation metric, assessing the trustworthiness of the sender based on their complaint patterns rather than solely on content analysis.
• Score range and interpretation: The BCL score ranges from 0 to 9. A score of 6 indicates a "mixed" complaint rate, implying that the sender is not consistently clean but also not a blatant spammer.
• Filtering mechanism: The BCL is used by Microsoft's anti-spam filters to determine how aggressively a message should be filtered, with higher scores leading to more stringent handling.
• Authentication impact: While not a direct BCL factor, proper email authentication (SPF, DKIM, DMARC) is crucial for identifying legitimate senders and building a reputation that contributes to lower BCL scores. See our simple guide to DMARC, SPF, and DKIM.

Key considerations

• Adherence to sender guidelines: Documentation recommends that senders comply with Microsoft's general sender guidelines, which include respecting user preferences and avoiding practices that lead to complaints. For more information, read how to comply with Outlook's new sender requirements.
• Feedback loop utilization: Official documentation encourages senders to use feedback loops to receive notifications about spam complaints and take prompt action, as this directly impacts BCL.
• Recipient engagement signals: While BCL is complaint-centric, positive engagement signals (opens, clicks) indirectly support a good sender reputation, which can mitigate negative BCL impacts by signaling legitimate sending practices.
• Header analysis: Documentation often provides guidance on interpreting email headers, including the BCL and other anti-spam stamps, to help senders diagnose and troubleshoot deliverability issues. The Microsoft documentation on anti-spam message headers is a key resource.