What factors influence a BCL 6 score in Outlook email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 8 Jul 2025
Updated 19 Aug 2025
8 min read
Understanding why your emails receive a Bulk Complaint Level (BCL) score of 6 in Outlook email deliverability is crucial for maintaining good inbox placement. A BCL score is assigned by Microsoft to incoming emails, indicating how likely a message is to be considered bulk or spam. The scale ranges from 0 to 9, where higher numbers suggest a greater likelihood of the email exhibiting undesirable spam-like behavior.
A BCL score of 6 means that an email is generating a mixed number of complaints, placing it firmly in a concerning category. While not immediately blocked, emails with a BCL of 6 are at significant risk of landing in junk folders or being quarantined rather than reaching the recipient's inbox.
This score directly influences how Outlook treats your messages. Identifying the specific factors contributing to a BCL 6 is key to improving your email deliverability and ensuring your communications reach their intended audience. It often comes down to a combination of sender reputation, content quality, and recipient engagement patterns.
Understanding BCL scores and their impact
A BCL score of 6 indicates that Microsoft's email filtering systems have detected a moderate to high level of complaints associated with your email sending. This score is a critical component alongside the Spam Confidence Level (SCL) in determining inbox placement for Outlook users. While SCL focuses on the likelihood of an email being spam based on its content, BCL specifically gauges the volume of complaints from recipients who mark your messages as unwanted bulk email.
The impact of a BCL 6 is straightforward: it significantly increases the chance of your emails bypassing the inbox and landing directly in the junk or spam folder. This can severely hinder your communication efforts, whether for marketing campaigns, transactional emails, or internal communications. If recipients frequently mark your messages as junk, Outlook learns from this behavior and applies a higher BCL to subsequent emails from your sending infrastructure.
Understanding how BCL and SCL ratings affect email deliverability is the first step toward diagnosis and remediation. While a BCL of 6 is problematic, it's often reversible with targeted efforts to improve sender reputation and recipient engagement. It signals that while you might not be a malicious sender, your sending practices are causing enough recipient dissatisfaction to trigger a notable complaint level.
Key factors influencing your BCL 6 score
Several critical elements contribute to your BCL score, with IP address reputation often being a primary driver. Microsoft closely monitors the sending behavior associated with your IP address. A history of high complaint rates, sending to unknown users, or engagement with spam traps from a particular IP can quickly lead to a higher BCL.
Your domain reputation is also a significant factor. Even if your IP is clean, a domain associated with previous spam complaints, low engagement, or poor authentication practices can negatively impact your BCL. Both your sending domain (the 'From' address) and your return-path (or Mail-From) domain are evaluated. Consistent poor performance from either can contribute to a BCL of 6.
Direct recipient complaints are the most explicit signal to Microsoft that your mail is unwanted. When recipients click the 'Junk' or 'Report Spam' button in Outlook, these complaints are tallied and directly contribute to your BCL. Even a small percentage of complaints can have a disproportionate impact, especially if you are sending large volumes of email. Keeping your complaint rate as low as possible is paramount for maintaining a healthy BCL and avoiding blocklists (or blacklists).
These three factors—IP reputation, domain reputation, and direct recipient complaints—are interconnected. A dip in one area can often drag down the others, making a holistic approach to email deliverability essential. For instance, sending emails that generate high complaints will quickly damage both your IP and domain reputation.
Content and sending practices
Beyond sender reputation, the actual content of your emails and your overall sending practices play a significant role in your BCL score. Emails containing characteristics common to spam messages can trigger filters even before explicit user complaints. This includes overly promotional language, excessive use of capitalization or exclamation marks, broken HTML, or suspicious links.
Poor list hygiene is a major contributor to high complaint rates and, consequently, a higher BCL. Sending to old, unengaged, or purchased lists often means hitting invalid email addresses, spam traps, or recipients who simply don't remember opting in. Spam traps are particularly damaging, as they are specifically designed to catch senders with bad list acquisition or management practices. Regularly cleaning your email lists and maintaining strict opt-in processes are fundamental to prevent a high BCL.
The volume and regularity of your sending patterns also matter. Sudden, uncharacteristic spikes in email volume from a previously low-volume sender can be flagged as suspicious. Email service providers (ESPs) and ISPs prefer consistent, predictable sending behavior, especially for bulk mailers. Irregular sending patterns, or sending large numbers of emails to recipients who haven't received mail from you in a long time, can lead to increased complaints and a worse BCL.
Formatting: Poorly formatted HTML, image-only emails, or hidden text.
Links: Use of redirect services, shortened URLs, or links to untrustworthy domains.
Technical authentication and user engagement
While not directly part of the BCL calculation itself, robust email authentication is foundational for overall deliverability and indirectly influences how Outlook perceives your legitimacy. Proper implementation of SPF, DKIM, and DMARC demonstrates that your emails are authentic and come from a verified sender, reducing the likelihood of them being flagged as suspicious, which could then lead to a higher BCL through increased complaints.
User engagement is perhaps the most significant, overarching factor influencing your BCL score. Outlook (and other ISPs) track how recipients interact with your emails. Positive engagement signals, such as opens, clicks, replies, and adding your address to their contacts, indicate that your mail is wanted. Conversely, negative engagement signals, like deleting emails without opening, moving them to junk, or unsubscribing, significantly harm your sender reputation and drive up your BCL.
To improve engagement and reduce BCL, focus on sending relevant content to an engaged audience. Implement double opt-in for new subscribers, regularly clean your lists of inactive users, and personalize your emails. A high BCL indicates recipients perceive your mail as bulk. By focusing on quality over quantity and prioritizing recipient experience, you can naturally lower your BCL and enhance your deliverability, steering clear of unwanted blocklists or spam folders.
Positive engagement
Opens: Recipients consistently opening your emails.
Clicks: Users clicking on links within your emails.
Replies/Forwards: Active interaction with your messages.
Marking as not junk: Moving emails from junk to the inbox.
Negative engagement
Spam complaints: Recipients reporting your email as spam or junk.
Unsubscribes: Opting out of your mailing list.
Low open rates: Emails frequently being ignored or deleted without opening.
Bounces: Sending to invalid or non-existent email addresses.
Improving your BCL score
Achieving a BCL 6 is a clear signal that Outlook considers your mail to be unwanted bulk. The factors influencing this score are primarily your IP and domain reputation, heavily driven by recipient complaints and overall engagement. Content quality, list hygiene, and consistent sending practices are also crucial.
To improve your BCL, focus on delivering highly relevant content to an engaged audience, maintaining impeccable list hygiene, and ensuring all email authentication protocols like DMARC, SPF, and DKIM are correctly configured. Proactive monitoring of your sender reputation will help you address issues before they escalate, ensuring your emails consistently reach the inbox.
Views from the trenches
Best practices
Actively monitor your IP and domain reputation. Look for changes in complaint rates and adjust sending volume accordingly.
Segment your email lists based on engagement. Prioritize sending to your most engaged subscribers to maintain positive signals.
Implement a double opt-in process for all new subscribers. This verifies intent and reduces complaints.
Provide a clear, easy-to-find unsubscribe link in every email.
Common pitfalls
Sending to purchased or old email lists without re-engagement campaigns, leading to high bounce and complaint rates.
Ignoring negative feedback loops from ISPs, which provide crucial data on why recipients mark your emails as spam.
Failing to implement or properly configure email authentication protocols (SPF, DKIM, DMARC).
Sending inconsistent email volumes, especially sudden spikes, which can trigger spam filters.
Expert tips
Sender reputation is paramount. A BCL 6 means you're crossing a threshold. Pay attention to engagement metrics.
Look for trends in your complaint rates. A sudden increase often points to a specific campaign or list issue.
The content of your email, beyond just keywords, plays a role. Assess readability, image-to-text ratio, and overall design.
Even if your authentication is technically correct, if users don't want your emails, your BCL will suffer.
Expert view
Expert from Email Geeks says that BCL is primarily driven by IP reputation. If your BCL is high, the first place to investigate is your sending IP.
2023-07-06 - Email Geeks
Marketer view
Marketer from Email Geeks says that a BCL of 6 indicates that the sender is generating a mixed number of complaints, signaling a need for immediate attention to their sending practices.
Microsoft to incoming emails, indicating how likely a message is to be considered bulk or spam. The scale ranges from 0 to 9, where higher numbers suggest a greater likelihood of the email exhibiting undesirable spam-like behavior.
