When you're troubleshooting email deliverability, especially with Microsoft services like Outlook or Office 365, delving into email headers becomes essential. These headers contain a wealth of information, providing a trail of how an email traveled from sender to recipient.
Among the various entries, one that frequently sparks curiosity is the X-Forefront-Antispam-Report header, and within it, the EFV tag. Understanding these headers is a key part of how to interpret Microsoft email headers and diagnose deliverability challenges.
While some parts of Microsoft's anti-spam headers are clearly documented, others, including EFV, remain a bit enigmatic. We'll explore what EFV likely signifies and why it's typically for internal diagnostic use by Microsoft.
Decoding Microsoft email headers
Email headers are like a digital passport for an email message, detailing its journey and the various checks it underwent. For Microsoft email environments, the X-Forefront-Antispam-Report header is particularly informative about spam filtering. This header includes numerous parameters that Microsoft's Exchange Online Protection (EOP) and Defender for Office 365 use to assess an email's legitimacy.
While values like SCL (Spam Confidence Level) and BCL (Bulk Complaint Level) are well-documented and directly indicate Microsoft's spam assessment, EFV is not. Microsoft itself states that some fields in this header are used exclusively by the Microsoft anti-spam team for diagnostic purposes.
This means that while we can observe EFV in email headers, its precise internal logic and direct correlation to deliverability outcomes are not publicly disclosed. It's likely a granular internal flag that contributes to, but doesn't solely determine, the overall spam score.
What EFV:NLI and SPM might mean
Despite the lack of official documentation, observations from the email community provide some insight into what EFV values like NLI or SPM might imply. The most commonly seen value is EFV:NLI;, which is often interpreted as NotLIsted. This suggests that the email hasn't triggered a specific blocklist (or blacklist) or internal flag related to EFV.
Another speculated value is EFV:SPM;, which could mean Spam. This is analogous to other known parameters like IPV:NLI (IP Not Listed) or SFV:NLI (Sender Filter Verdict Not Listed). These tags, while not fully explained, provide clues about different layers of Microsoft's filtering processes.
Observed meaning
NLI: Indicates the email is not listed on a particular internal list or check related to the Email Filtering Verdict. This is generally a neutral or positive sign.
SPM: Speculated to indicate a Spam verdict for this specific filter. This would likely contribute negatively to the overall spam score.
These interpretations, though unofficial, help fill the gaps in understanding Microsoft's complex anti-spam system. For a deeper dive into Microsoft's approach to anti-spam, reviewing their new sender requirements can be beneficial, as they outline crucial steps for maintaining good sender reputation.
Impact on deliverability
While understanding EFV provides a piece of the puzzle, it's generally understood not to be a direct indicator of inbox placement outcomes. Insights from Microsoft support suggest that this particular value doesn't influence whether an email lands in the inbox or the junk folder. This means that while EFV is a diagnostic flag, it's not the primary lever for deliverability.
Focusing too much on undocumented headers like EFV can distract from more critical factors affecting your email's journey. Instead, attention should be directed towards well-known signals that Microsoft uses, such as SPF, DKIM, and DMARC authentication, and maintaining a positive sender reputation. These elements are far more influential in determining where your emails land.
If you're facing deliverability issues, exploring other parts of the X-Forefront-Antispam-Report header, particularly the SCL (Spam Confidence Level) and BCL (Bulk Complaint Level) values, will provide more actionable insights. You can also gain insight from understanding the Mailbox-Delivery header values.
Beyond EFV: what to look for
Since EFV is primarily for internal diagnostics, our efforts are best spent on factors that we know directly influence deliverability. Here's what to prioritize when troubleshooting Microsoft email delivery.
Authentication: Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned. Email authentication is fundamental for proving your legitimacy.
Sender reputation: Monitor your sender reputation using tools like Outlook's Postmaster Tools. A poor reputation leads to junk folder placement, or even being added to a blocklist.
Content quality: Avoid spammy content, excessive links, or suspicious attachments that could trigger filters. This is one of the key reasons emails go to spam.
List hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing bounce rates and spam trap hits.
While we can't fully decode every diagnostic flag in Microsoft email headers, focusing on these established best practices will yield far better results for your email deliverability.
Analyzing the X-Forefront-Antispam-Report header
To effectively troubleshoot, it's helpful to know how to determine an email sending platform from email headers. Here's an example of how the X-Forefront-Antispam-Report header might appear, showcasing different diagnostic flags that offer more actionable insights than EFV.
In this example, while EFV:NLI; is present, the more significant indicators are SCL:5 and BCL:7. An SCL of 5 suggests a high probability of spam, and a BCL of 7 indicates a significant bulk email score, which is also a strong negative signal. These are the values that truly dictate whether your email lands in the inbox or the junk folder. You can see more about this in our article on understanding email headers.
Understanding what each element means in a header can significantly improve your troubleshooting process. While EFV remains somewhat obscure, focusing on the well-documented headers and their corresponding scores will lead to more effective solutions for your deliverability challenges.
Views from the trenches
Best practices
Always prioritize your SPF, DKIM, and DMARC alignment and make sure these are correctly configured.
Regularly monitor your domain and IP reputation using available postmaster tools.
Maintain clean email lists to prevent sending to spam traps and invalid addresses.
Focus on email content quality and personalization to reduce spam filter triggers.
Common pitfalls
Over-analyzing undocumented or internal-only email header fields like EFV.
Ignoring common deliverability metrics like SCL and BCL in favor of obscure flags.
Failing to implement or properly configure DMARC, SPF, and DKIM.
Sending emails to unengaged recipients or purchased lists, damaging sender reputation.
Expert tips
If you suspect a Microsoft-specific issue, check their official documentation for header explanations.
Engage with the email community on forums to gather insights on undocumented flags.
Utilize DMARC reports to identify authentication failures and potential spoofing attempts.
Implement a feedback loop service to track user complaints directly from ISPs.
Marketer view
Marketer from Email Geeks says they received an answer from Microsoft support stating that EFV doesn't influence inbox placement, and no further details could be provided.
2019-11-15 - Email Geeks
Marketer view
Marketer from Email Geeks says that the :NLI; part likely means 'NotLIsted', and :SPM could mean 'Spam'. They noted that EFV might stand for Email Filtering Verdict, but that's an educated guess.
2019-11-15 - Email Geeks
The broader context of email deliverability
While the EFV tag in Microsoft email headers remains largely undocumented and primarily for internal diagnostic use, it's a small piece of a much larger and more transparent system. Email deliverability is a complex field, and while every detail in a header can spark curiosity, not all of them hold direct keys to improving your inbox placement. The critical takeaway is to focus on what we know works, rather than getting lost in obscure technical flags.
Outlook's Postmaster Tools. A poor reputation leads to junk folder placement, or even being added to a blocklist.
