Suped

Summary

The EFV header in Microsoft email headers is primarily an internal diagnostic field used by Microsoft's anti-spam team. Its exact meaning is not publicly documented, and Microsoft explicitly states that such fields are for their exclusive diagnostic purposes. While the :NLI component is commonly speculated to mean Not Listed, the full abbreviation EFV remains unofficial. Understanding this header is more about curiosity than direct actionable insights for email deliverability, as it does not typically influence inbox placement.

What email marketers say

Email marketers often dive deep into headers to understand deliverability issues, but the EFV header from Microsoft remains a topic of curiosity rather than a reliable diagnostic tool. Most marketers acknowledge its existence but find its utility limited due to the lack of official documentation from Microsoft. They frequently resort to community discussions and educated guesses to interpret its meaning, confirming that it doesn't seem to directly impact inbox placement.

Marketer view

Email marketer from Email Geeks explains that the last answer received from Microsoft support indicated that the EFV header does not influence inbox placement, and no further details could be provided, suggesting it's an internal metric.

15 Nov 2019 - Email Geeks

Marketer view

Marketer from Spiceworks Community notes that when facing junk mail issues, the EFV header is typically not the primary clue, implying other factors are more significant for diagnosis.

20 Jun 2018 - Spiceworks Community

What the experts say

Email deliverability experts, with their deep understanding of mail systems and anti-spam mechanisms, generally concur that the EFV header is a Microsoft internal artifact. They advise senders not to over-analyze such undocumented headers for deliverability troubleshooting. Instead, they recommend focusing on well-understood authentication standards and sender reputation metrics that demonstrably impact inbox placement.

Expert view

Expert from Email Geeks explains that any definitive answer about the EFV header's influence on inbox placement would require direct confirmation from Microsoft's internal teams, which is rarely provided for such fields.

15 Nov 2019 - Email Geeks

Expert view

Expert from Word to the Wise advises that senders should focus their efforts on clearly defined and actionable deliverability metrics rather than getting bogged down in interpreting proprietary, undocumented headers.

01 Oct 2023 - Word to the Wise

What the documentation says

Microsoft's official documentation on anti-spam message headers offers insights into various fields that indicate how an email has been processed. However, for fields like EFV, the documentation explicitly states that they are "used exclusively by the Microsoft anti-spam team for diagnostic purposes." This clearly establishes their proprietary and internal nature, limiting the extent to which senders can use them for their own deliverability analysis.

Technical article

Microsoft Documentation states that X-Forefront-Antispam-Report contains several fields, and EFV is among those used exclusively by their anti-spam team for diagnostic purposes.

15 Jan 2024 - Microsoft Docs

Technical article

GitHub Issue related to Microsoft documentation highlights that EFV:NLI is a common sighting in headers but lacks a specific definition in the official anti-spam header documentation, leading to user queries.

10 Mar 2020 - GitHub

3 resources

Start improving your email deliverability today

Get started