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What is header.i in email headers, how does it relate to DKIM, and does it affect sender reputation?

Summary

The header.i tag in an email header, specifically within the DKIM (DomainKeys Identified Mail) signature, serves as an identifier for the agent or user responsible for signing the email. While header.from represents the visible sender address (the RFC 5322 From header), header.i (or i= in the DKIM-Signature) points to the signing identity, often a subdomain or specific email address under the primary d= (domain) tag. This distinction is crucial for email authentication protocols like DKIM, which help verify sender authenticity and prevent spoofing.

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What email marketers say

Email marketers often encounter complexities within email headers when troubleshooting deliverability issues. The relationship between header.i, header.from, and the overall DKIM signature can be a source of confusion, particularly when trying to understand how different domains interact and affect sender reputation.

Marketer view

Email marketer from Email Geeks indicates they are trying to understand the discrepancy between header.i being the sending domain and header.from being the organizational domain. This suggests a common challenge in interpreting complex email headers for authentication purposes.

14 Aug 2019 - Email Geeks

Marketer view

Email marketer from Email on Acid highlights that a DKIM signature helps mailbox providers verify the sender and prevent phishing attacks like email spoofing. This underscores DKIM's foundational role in email security and deliverability.

15 Sep 2023 - Email on Acid

What the experts say

Email deliverability experts typically view header.i as an important, albeit secondary, component of the DKIM authentication process. While the d= tag (signing domain) is paramount, header.i (identity) provides granular detail about the specific entity within that domain that initiated the email. Experts emphasize that the core purpose of header.i is for internal accountability, rather than a primary external reputation factor, although its consistency and proper alignment can still play a role.

Expert view

Expert from Email Geeks indicates that the 'i=' tag within a DKIM signature identifies a more specific identity responsible for the signature. This identity can be either an email address or a subdomain of the 'd=' domain, providing granular detail on the signing entity.

14 Aug 2019 - Email Geeks

Expert view

Expert from Word to the Wise explains the difference between DKIM's 'i=' and 'd=' tags, noting that 'i=' allows a more specific identity to take responsibility for the signature. This distinction is crucial for understanding how DKIM signatures attribute responsibility for email authentication.

29 Oct 2007 - Word to the Wise

What the documentation says

Official documentation and standards, such as those from the IETF (Internet Engineering Task Force) that define DKIM, clarify the role of the header.i tag. The i= tag, or 'identity', specifies the entity within the signing domain responsible for the message. While it's optional, its presence helps in granular attribution of responsibility for the DKIM signature.

Technical article

Documentation from Mailgun states that email authentication protocols like SPF, DKIM, and DMARC are crucial for verifying sender identity, preventing spoofing, and supporting a strong sender reputation. These protocols work in concert to establish trust in the email ecosystem.

10 Aug 2023 - Mailgun

Technical article

Documentation from Mailjet highlights that email headers help verify message security during transit and confirm it reached the recipient without errors or alterations. This underscores the header's role as a diagnostic tool for delivery and integrity issues.

10 Apr 2025 - Mailjet

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