The DMARC adkim tag is a critical component within your DMARC record that dictates how strictly DKIM authentication must align with your organizational domain. Email authentication protocols like DKIM (DomainKeys Identified Mail) are foundational for proving that an email sender is legitimate and authorized to send mail on behalf of a domain. Without proper alignment, even successfully authenticated emails might fail DMARC checks, leading to deliverability issues.
When an email is sent, it goes through several authentication checks, including SPF and DKIM. DMARC (Domain-based Message Authentication, Reporting, and Conformance) then builds on these by requiring that the domains used in the From header of the email align with the domains that passed SPF or DKIM. The adkim tag specifically defines the required DKIM alignment mode.
Correctly configuring the adkim tag is crucial for achieving strong DMARC enforcement and preventing email impersonation. It directly impacts how mail receivers evaluate your emails' authenticity, influencing whether they reach the inbox or are marked as spam. Effective DMARC deployment, including careful consideration of adkim, is vital for maintaining a good sender reputation and ensuring your messages are delivered reliably.
Understanding DKIM alignment
DKIM alignment is the process by which DMARC verifies that the domain used for DKIM signing (the d= tag in the DKIM signature) matches the organizational domain found in the email's From header. This check is crucial because it ensures that the domain claiming responsibility for sending the email is indeed the one being protected by DMARC. Without this alignment, an attacker could sign an email with DKIM using a different domain, and it might still pass DKIM authentication, but it would fail DMARC.
There are two modes of alignment, relaxed and strict, which determine how exact this match needs to be. Understanding dkim-alignment is key to successful DMARC implementation. For instance, if your From header domain is yourdomain.com, the DKIM signature's d= tag also needs to reflect yourdomain.com or a subdomain of it, depending on your chosen alignment mode. This process is similar to how aspf works for SPF alignment.
This alignment ensures that the reputation of the domain in the From header, which is what end-users see, is directly tied to the authentication results. This direct link helps receiving mail servers make informed decisions about whether to trust an incoming email, thus protecting your brand from phishing and spoofing attacks. You can learn more about general DMARC records and their tags through resources like Mailjet's DMARC record guide.
The 'adkim' tag and its values
The adkim tag in a DMARC record specifies the DKIM identifier alignment mode. It determines the criteria for matching the DKIM signing domain (found in the DKIM d= tag) with the organizational domain in the email's From header. There are two possible values for the adkim tag: r for relaxed mode and s for strict mode.
In relaxed mode (adkim=r), the DKIM d= domain can be an exact match or a subdomain of the From header's organizational domain. This flexibility is often useful for organizations that send emails through third-party services that sign emails with their own subdomains, such as someservice.yourdomain.com. It allows these emails to pass DMARC as long as the base domain matches. This mode is generally easier to implement and provides a broader range of compliant sending scenarios.
Conversely, strict mode (adkim=s) demands an exact match between the DKIM signing domain and the From header's organizational domain. This means that if your From header is yourdomain.com, the DKIM d= tag must also be yourdomain.com, not a subdomain. This provides a stronger level of security, making it harder for attackers to spoof emails. You can find more details about adkim=s in our knowledge base.
Comparing relaxed and strict alignment
Relaxed alignment (adkim=r)
Flexibility: Allows subdomains to align with the root domain. For example, email.yourdomain.com can align with yourdomain.com.
Third-party sending: Ideal for organizations using third-party email service providers (ESPs) that sign with a subdomain.
Implementation: Easier to deploy and less prone to breaking legitimate email flows, especially during initial DMARC setup.
Security trade-off: Offers slightly less stringent protection against highly sophisticated spoofing attacks compared to strict mode.
Strict alignment (adkim=s)
Exact match: Requires the DKIM signing domain to be an identical match to the From header's organizational domain.
Enhanced security: Provides the highest level of protection against direct domain spoofing and phishing.
Less flexible: May require more complex configuration for third-party senders, who might need custom DKIM keys.
Best for controlled environments: Ideal for organizations with tight control over all email sending infrastructure.
Choosing the right alignment mode
While adkim=s offers superior security, adkim=r is often recommended as a starting point, especially for organizations with complex email infrastructure or numerous third-party senders. Transitioning from relaxed to strict alignment should be done carefully after ensuring all legitimate sending sources can meet the stricter requirements. Always monitor your DMARC reports closely during this process.
Many organizations begin with adkim=r and potentially transition to adkim=s once they have a complete overview of their email ecosystem and have configured all sending sources to comply with the stricter standard. This iterative approach minimizes the risk of legitimate emails failing DMARC and being rejected or quarantined.
Impact on DMARC policy enforcement
The adkim tag plays a pivotal role in how your DMARC policy is applied. If an email fails DKIM authentication or DKIM alignment according to the adkim setting, it will fail the DMARC check for DKIM. For an email to pass DMARC, at least one of SPF or DKIM must pass authentication AND alignment. This means a misconfigured adkim tag could lead to legitimate emails being treated as suspicious.
When an email fails DMARC, the receiving server will take action based on the p tag in your DMARC record. This policy (e.g., p=none, p=quarantine, or p=reject) determines whether the email is delivered to the inbox, sent to spam, or rejected entirely. This means that a seemingly minor setting like adkim can have a significant impact on your email deliverability and overall brand reputation.
Monitoring your DMARC reports is essential to understand the impact of your adkim setting. Tools like Suped's DMARC monitoring provide granular insights into your email traffic, showing you which emails pass or fail DMARC and why. These insights help you identify legitimate sending sources that might be failing DKIM alignment and adjust your DMARC record or sending practices accordingly to prevent emails from going to spam or being blocked (or blacklisted). Suped offers AI-powered recommendations, real-time alerts, and a unified platform for comprehensive email security and deliverability management.
Implementing 'adkim' in your DMARC record
Configuring the adkim tag involves adding it to your DMARC DNS TXT record. This record is typically located at _dmarc.yourdomain.com. The value you choose for adkim (either r for relaxed or s for strict) should be based on your email sending architecture and your desired level of security versus flexibility.
For domains with a well-controlled sending environment where all legitimate emails are signed with the exact organizational domain, adkim=s offers the strongest protection. However, many organizations find adkim=r to be a more practical initial choice, especially when working with various third-party email services. Always remember to test thoroughly and analyze your DMARC reports when making changes to this tag to avoid unintended email delivery failures.
Enhancing email authentication and deliverability
The DMARC adkim tag is a powerful lever for controlling the strictness of your DKIM alignment, directly impacting your email security and deliverability. By choosing between relaxed (adkim=r) and strict (adkim=s) modes, you can fine-tune how aggressively DMARC protects your domain from unauthorized senders, while ensuring your legitimate emails are not inadvertently rejected.
Effective DMARC implementation is an ongoing process that requires continuous monitoring and adjustments. Regular review of your DMARC reports, along with other email authentication and deliverability metrics, is crucial for maintaining optimal email performance. With the right configuration of the adkim tag and a robust DMARC monitoring solution like Suped, you can significantly enhance your email security posture and ensure your messages consistently reach their intended recipients.
