When you delve into the technical side of email authentication, you often encounter various tags within DKIM signatures. One of these, often a source of confusion, is the 'i' tag. It plays a specific, though sometimes overlooked, role in identifying the agent or user responsible for signing an email.
The purpose of DKIM is to allow the receiver to verify that an email was indeed sent by an authorized sender for that domain. It essentially provides a way for a domain to assert responsibility for an email message, preventing email spoofing and phishing attacks. While the 'd=' tag specifies the domain responsible for the signature, the 'i=' tag offers a more granular level of identification.
Understanding this tag is crucial for comprehensive email authentication. It helps clarify who or what within a domain is actually originating the message, which can be particularly important for larger organizations or those using third-party sending services. Let's break down what the 'i' tag is, why it matters, and how to manage it effectively.
The identifier for the signing entity
In a DKIM signature, the 'i' tag, or Agent or User Identifier, specifies the full email address of the agent or user on behalf of whom the signing domain is claiming responsibility. Essentially, it identifies the specific entity that initiated the email within the broader signing domain. This is distinct from the d= tag, which indicates the domain doing the signing.
While the 'd' tag in a DKIM signature points to the domain responsible for signing the message, the 'i' tag offers a more precise identifier. Think of the 'd' tag as the company (e.g., example.com) and the 'i' tag as a specific department or individual within that company (e.g., marketing@example.com). This distinction is outlined in RFC 6376, the specification for DKIM.
Example DKIM-Signature Header with 'i' tagemail-header
While the 'i' tag typically contains an email address, it doesn't necessarily have to be a functional mailbox. It acts as an identifier for the organizational unit or sender responsible for the email. Often, if omitted, the 'i' tag defaults to the domain specified in the 'd' tag, creating a simpler scenario where the signing domain takes responsibility for all emails from that domain.
Impact on sender reputation and DMARC alignment
The 'i' tag can influence sender reputation, especially when a domain is delegating email sending to a third party. When the 'i' tag (agent or user identity) is a subdomain of the 'd' tag (signing domain), it helps maintain strong domain alignment, which is crucial for DMARC authentication. If these tags are misaligned or used incorrectly, it can lead to DKIM failures and negatively impact email deliverability.
Mailbox providers (like Gmail and Outlook) use DKIM (among other protocols like SPF) to verify email legitimacy. When the 'i' tag's domain doesn't align with the header From address, it can signal a potential threat, leading to emails being sent to spam or even being blocked entirely. This is why consistent and proper DKIM configuration, including the 'i' tag, is vital for ensuring your emails reach the inbox. Poor configuration can result in your domain ending up on a email blocklist (or blacklist).
DMARC monitoring with Suped
Monitoring your DKIM alignment and DMARC results is essential. Suped offers DMARC monitoring with AI-powered recommendations to identify and fix issues, ensuring strong authentication and improving your email deliverability. Our platform provides real-time alerts and a unified dashboard for managing all your email authentication needs, including Suped's SPF flattening functionality, to help you stay compliant.
As noted by Word to the Wise, the 'i=' tag is akin to a 'MySender' header, adding an extra layer of data about the sender within the 'd=' domain. For DMARC, if the domain in the 'i=' tag is the same as, or a subdomain of, the RFC5322.From domain, it contributes to DKIM alignment. Without this alignment, even a valid DKIM signature might not pass DMARC, potentially leading to deliverability problems.
Configuration details and best practices
The 'i' tag is an optional component in a DKIM signature, meaning it can be present or absent. When it's absent, the signing domain ('d' tag) is considered the sole identifier. When present, it provides more specific attribution. For most organizations sending their own emails directly, the 'i' tag is often omitted or set to match the 'd' tag for simplicity and strong alignment. However, in complex setups, it can serve a valuable purpose.
Best practice dictates that if the 'i' tag is used, its domain part should be the same as or a subdomain of the 'd' tag. This ensures consistency and strengthens trust signals for receiving mail servers. Mismatches here can trigger flags and lead to emails being treated as suspicious. For example, if your 'd' tag is yourdomain.com, an 'i' tag of user@yourdomain.com or user@mail.yourdomain.com would be ideal for alignment.
Correct 'i' tag usage
Domain Alignment: The 'i' tag's domain matches or is a subdomain of the 'd' tag and the RFC5322.From domain.
Explicit Identification: Used when granular attribution is needed, for example, identifying specific senders within an organization.
Third-Party Senders: When using a third-party email service, ensuring the 'i' tag aligns correctly with your domain strategy helps maintain trust.
Incorrect 'i' tag usage
Domain Mismatch: The 'i' tag's domain is completely different from the 'd' tag or RFC5322.From domain, leading to DMARC failure.
Generic Use: Using a generic 'i' tag when more specific identification could improve reputation management.
Overly Complex: Introducing unnecessary complexity with the 'i' tag when the 'd' tag alone would suffice, increasing error potential.
While DKIM itself is about domain authentication, the 'i' tag provides an additional layer that can be useful for internal tracking and more precise attribution. If you're managing complex sending infrastructures, understanding this nuance is key to optimizing your overall email security and deliverability strategy. Regularly reviewing your DKIM records and reports can help catch any misconfigurations involving the 'i' tag early on.
Troubleshooting 'i' tag issues
Troubleshooting issues related to the 'i' tag often boils down to checking for proper alignment and consistent usage across your email streams. A common problem arises when the domain specified in the 'i' tag doesn't align with the domain in the 'From' header, which directly impacts DMARC policy enforcement. This can mistakenly mark legitimate emails as unauthenticated, leading to delivery failures.
Scenario
DKIM 'i' Tag Value
DMARC Alignment Outcome
Perfect alignment
user@yourdomain.com
Passes DMARC DKIM alignment
Subdomain alignment
user@sub.yourdomain.com
Passes DMARC DKIM alignment (relaxed mode)
Omitted 'i' tag
N/A (defaults to d= domain)
Passes DMARC DKIM alignment if d= aligns
Domain mismatch
user@anotherdomain.com
Fails DMARC DKIM alignment
If you are experiencing email deliverability issues, checking the 'i' tag's configuration should be part of your diagnostic process. Ensure that the 'i' tag's domain is always aligned with the 'd' tag and the visible 'From' address. If you use a third-party sender, make sure they configure the 'i' tag appropriately on your behalf, often by using a subdomain they control or by omitting it if their signing domain (d=) aligns correctly with your From header. This careful attention to detail helps prevent your emails from being mistaken for spam.
Ensuring proper DKIM configuration
The 'i' tag in a DKIM signature, while optional, serves as a more specific identifier for the entity responsible for signing an email. Its correct configuration, particularly its alignment with the 'd' tag and the email's 'From' address, is crucial for maintaining strong email authentication and ensuring deliverability. Mismatches can lead to DMARC failures and negatively impact your sender reputation, potentially leading to your domain being placed on a email blocklist or blacklist. By paying attention to these details, you can significantly enhance your email security posture.
Consistently monitoring your DKIM and DMARC reports is the best way to catch any misconfigurations or issues related to the 'i' tag or other authentication elements. Platforms like Suped provide detailed insights and actionable recommendations, making it easier to manage your email authentication protocols. A well-configured DKIM, with proper attention to the 'i' tag when used, is a cornerstone of effective email communication.
with AI-powered recommendations to identify and fix issues, ensuring strong authentication and improving your email deliverability. Our platform provides real-time alerts and a unified dashboard for managing all your email authentication needs, including Suped's SPF flattening functionality, to help you stay compliant.
