DKIM key rotation is primarily a security best practice, but it has significant indirect implications for email deliverability. While non-rotation doesn't inherently harm deliverability, a compromised or old key can lead to authentication failures, increasing the likelihood of emails landing in spam or being outright blocked. Regular rotation mitigates this risk by reducing the window of vulnerability and ensuring that, in the event of a breach, there are established procedures and knowledgeable personnel to execute a smooth key change without disrupting email flow.
Key findings
Security first: DKIM key rotation is fundamentally a security measure designed to protect against key compromise and replay attacks, as outlined by authoritative bodies like M3AAWG in their Best Current Practices on DKIM Key Rotation.
Indirect deliverability impact: While merely not rotating keys does not directly lower deliverability, a compromised key can immediately and severely impact it by causing DKIM authentication to fail, leading to emails being flagged as suspicious or spam. This is why DKIM key rotation is recommended.
Operational readiness: Regular rotation ensures that the technical team maintains proficiency in the process, making it easier to execute an emergency key change in case of a compromise, minimizing potential downtime and deliverability issues.
Selector changes: Effective key rotation requires changing the DKIM selector with each new key pair. This allows for a graceful transition period where both old and new keys (via different selectors) can be active simultaneously, preventing email authentication failures for mail in transit. For more, read about how DKIM selectors impact email reputation.
ESP best practices: Many major ESPs and email providers (like Microsoft, Fastmail, Proton) automatically rotate keys and manage multiple selectors to ensure seamless transitions and maintain strong authentication.
Key considerations
Frequency: There isn't a universal consensus, but recommendations range from quarterly to every six months, or at least annually. The ideal frequency balances security benefits with operational effort.
Transition period: When rotating keys, it is crucial to keep the old public key in DNS for a period (e.g., a week or more) to allow for the delivery of emails signed with the previous key before it is removed. This prevents temporary authentication failures.
DNS management: For senders managing their own DKIM records, this means adding new TXT records with new selectors and eventually removing old ones. ESPs often simplify this through CNAME delegation or by managing multiple selectors automatically.
Impact on existing mail: Emails already sent and in transit will still be verified against the key that was active when they were signed. The selector ensures the correct key is found, even if a new one is introduced for new mail.
Importance of DMARC: With DMARC implemented, failed DKIM authentication (due to a compromised or improperly removed key) would cause mail to fail DMARC alignment, leading to rejection or quarantine based on your DMARC policy. This highlights the synergy between DKIM rotation and overall email authentication.
Email marketers generally agree that while DKIM key rotation isn't a direct dial for improving deliverability, it's a critical security practice that indirectly safeguards it. The consensus leans towards preventing future deliverability issues arising from compromised keys rather than seeing rotation as a proactive measure to boost inbox placement. Many acknowledge that consistent rotation is not a common practice among most senders, but it's essential for robust email security.
Key opinions
No direct deliverability boost: Many marketers haven't observed a direct improvement in deliverability metrics solely from rotating DKIM keys, suggesting the primary benefit is not about immediate inbox placement gains.
Security as a deliverability safeguard: The value of rotation is seen in its ability to prevent compromises that would, in turn, severely degrade deliverability. An insecure key is a ticking time bomb for your inbox placement.
Underutilized practice: There's an observation that DKIM key rotation is not widely adopted by email senders, despite its acknowledged security benefits. This suggests a gap in best practice implementation.
Long-term security: Marketers emphasize the long-term security benefits of rotation, comparing it to regular password updates for digital accounts. This preventative measure is considered essential for sustained email security. This helps to strengthen your domain reputation.
Operational know-how: Regular rotation ensures that the process and knowledge for changing keys are maintained within the organization, making it less likely to break email delivery during an unexpected emergency rotation. Read more about implementing DKIM, SPF, and DMARC.
Key considerations
Risk of compromise: Failing to rotate keys increases the risk of them being compromised over time, which is a direct pathway to deliverability failure.
Importance of selectors: Marketers should be aware that key rotation typically involves changing the DKIM selector to ensure a smooth transition and allow for the graceful deprecation of old keys.
ESP support: If using an ESP, understand their DKIM rotation policies and whether they automatically handle key rotation and selector management. Many ESPs pre-provision multiple selectors to facilitate this.
Time-sensitive delivery: The nature of email delivery means messages can be in transit for some time. This necessitates keeping old DKIM keys active for a transition period after a new key is introduced to avoid authentication failures.
Best practice vs. immediate impact: While immediate deliverability gains may not be apparent, adhering to key rotation as a security best practice is crucial for long-term email program stability and reputation, preventing unexpected drops in deliverability due to compromise.
Marketer view
Email Marketer from Email Geeks states that they have never observed any indication that DKIM key rotation directly improves email deliverability.
02 Feb 2024 - Email Geeks
Marketer view
Marketer from AutoSPF suggests rotating DKIM keys at least once every six months, noting that four rotations per year is an even safer choice if resources permit. This regular practice enhances security.
15 Mar 2024 - AutoSPF
What the experts say
Experts strongly advocate for DKIM key rotation as a fundamental security best practice. While they acknowledge that non-rotation itself doesn't directly harm deliverability, a compromised key poses a significant threat, capable of tanking deliverability overnight. They emphasize the importance of changing selectors during rotation to allow for a smooth transition and the need to keep old keys active for a period to account for email's asynchronous delivery. Operational readiness and established procedures are key benefits of consistent rotation.
Key opinions
Security is paramount: Experts confirm that DKIM key rotation is primarily a security recommendation. Compromised keys can lead to significant deliverability issues.
Uncommon practice: It is noted that many senders do not regularly rotate their DKIM keys, even though it is a recommended best practice by industry leaders like Fastmail.
Long-lived keys are risky: Active keys that have been in place for many years, (e.g., since 2006), pose a substantial risk. If such keys are ever cracked, deliverability for that sender will plummet immediately.
Operational preparedness: A key benefit of regular rotation is ensuring the company has personnel who know how to execute the rotation process smoothly when a compromise inevitably forces a key change. This preparedness helps prevent email disruptions and is a key factor in technical solutions for email deliverability.
Selector is essential: Experts affirm that a new selector must be used when rotating DKIM keys. This allows for both the old and new keys to be active simultaneously during the transition period, preventing authentication failures for emails already in transit. For practical examples, see DKIM selector name examples.
Temporary coexistence: Old DKIM keys should not be deleted immediately after a new key is introduced because emails take time to be delivered. A transition period is necessary to ensure successful authentication of all messages signed with the older key.
ESP methods: Many ESPs, including Microsoft, Fastmail, and Proton, follow best practices by rotating keys between different selectors (e.g., selector1 and selector2) and keeping old keys active for a period. This often involves CNAME delegation.
Key considerations
DNS propagation delays: Remember that DNS changes, including new DKIM records, can take time to propagate globally. This further necessitates the transitional period for old keys.
Automated vs. manual: While some large providers offer automated key rotation, many senders or smaller ESPs may need to manage this process manually, which requires careful planning and execution.
Operational overhead: Implementing regular key rotation adds an operational task, but experts suggest this overhead is justified by the security and long-term deliverability benefits it provides.
Impact on DMARC: Failed DKIM authentication due to a prematurely removed old key will cause messages to fail DMARC alignment, impacting the sender's reputation and potentially leading to rejection. Therefore, DKIM rotation must be done with DMARC reporting in mind.
Transition period length: The exact length of time to keep old keys active varies, but a week is often suggested as a safe minimum, depending on typical email delivery times and network conditions.
Expert view
Expert from Email Geeks indicates that they have seldom seen any evidence of widespread DKIM key rotation practices among email senders. This suggests that while it's a recommended security measure, its adoption may not be universal.
02 Feb 2024 - Email Geeks
Expert view
Expert from Spamresource asserts that regular key rotation is critical for security, and that security failures can undeniably impact deliverability. They emphasize that a strong security posture directly supports consistent inbox placement.
10 Mar 2024 - Spamresource
What the documentation says
Documentation consistently frames DKIM key rotation as a crucial security measure. It highlights that regular rotation minimizes the window of vulnerability for compromised keys, which, if exploited, could severely impact email authenticity and deliverability. Best practices from various platforms, including cloud email services, emphasize the use of multiple selectors and a transition period where both old and new keys are active, ensuring that emails in transit are still verifiable. This systematic approach is key to maintaining integrity and preventing service disruption.
Key findings
Security imperative: Official documentation strongly recommends DKIM key rotation as a vital security measure to maintain the integrity and security of email communications, preventing potential compromises that could lead to unauthorized email signing.
Risk mitigation: Regular rotation reduces the risk of key compromise, which, if it occurs, can directly lead to email authentication failures and impact deliverability. This practice is akin to frequently updating passwords for enhanced account security.
Selector utilization: DKIM selectors are instrumental in facilitating key rotation by allowing senders to transition smoothly between old and new keys without disrupting active email streams. This ensures seamless validation during the switchover.
Graceful transition: A common strategy highlighted in documentation is to publish a new key with a new selector while keeping the old key active for a period. This accommodates emails still in transit that were signed with the old key.
Automated rotation: Some major email platforms, such as AWS SES and Microsoft 365, feature automatic DKIM key rotation, often publishing multiple active keys or transitioning between selectors to ensure continuous service without manual intervention from users. For more details on Microsoft's approach, you can refer to relevant O365 documentation.
Key considerations
Recommended frequency: Recommendations for key rotation frequency vary, but typically suggest at least every six months, with some advocating for quarterly rotation as a robust security measure.
Preventing authentication failures: Documentation stresses that premature removal of old keys can cause verification failures for legitimate emails, underscoring the importance of a proper transition period.
Integration with DMARC: The efficacy of DKIM key rotation is often discussed in conjunction with DMARC, as DMARC relies on DKIM (and SPF) for alignment to determine the authenticity of a message. Proper rotation ensures DKIM continues to pass, supporting DMARC. Learn more in this guide to DMARC, SPF, and DKIM.
DNS record management: Senders must manage their DNS records carefully during rotation, adding new TXT records for new selectors and removing outdated ones only after a safe period has elapsed.
Key length: While not directly about rotation frequency, some documentation touches upon the recommended DKIM key length (e.g., 2048-bit) as a foundational security element that complements regular rotation for robust email authentication.
Technical article
Documentation from M3AAWG Best Current Practices for DKIM Key Rotation states that key rotation is a critical security measure aimed at reducing the potential impact of a compromised key. This proactive step minimizes the risk of unauthorized signing.
02 Feb 2024 - m3aawg.org
Technical article
Mailgun's documentation emphasizes that regular key rotation prevents malicious actors from exploiting older, potentially compromised keys. This security practice is crucial for maintaining the integrity of email authentication and trust.