Can old DKIM records from previous ESPs negatively impact email sending reputation?

Key findings

• DKIM visibility: DKIM records are only actively used by receiving mail servers when email is sent through the ESP associated with that specific DKIM key. If you're no longer sending from the old ESP, the old DKIM record will not be actively checked or impact your live sending.
• Reputation continuity: Spam filters do retain some memory of your sending domain's reputation. A DKIM signature helps them identify your mail stream regardless of the sending IP. Therefore, when you move to a new ESP, your existing domain reputation, established partly by your DKIM signature, will carry over for a period. This is part of how email reputation transfers during a migration.
• Domain reputation: A significant part of your domain's email reputation is tied to the domain (`d=`) in your DKIM signature. This reputation, rather than just the sending IP, becomes the primary factor for deliverability with advanced mailbox providers.
• Impersonation risk: While small, a dormant DKIM public key in your DNS records could pose a security risk if the corresponding private key were ever compromised. An attacker with the private key could theoretically sign emails as your domain, and these emails would pass DKIM validation.

Key considerations

• DNS tidiness: Deleting old, unused DKIM records is a good DNS hygiene practice. It simplifies management and reduces potential confusion, even if it doesn't directly harm deliverability.
• Security best practice: While the risk of a compromised private key for an unused DKIM record is low, removing it eliminates this potential vulnerability, enhancing your overall email security posture.
• DMARC monitoring: With an active DMARC policy, you can monitor for unexpected senders. If an old DKIM key were somehow misused, DMARC reports might show traffic from an unfamiliar source, even if it passes authentication, allowing you to investigate. More information can be found on email sender reputation.
• Migration strategy: Properly managing DNS records, including DKIM, is a critical part of a successful ESP migration strategy to ensure smooth deliverability during the transition.

Key opinions

• Historical impact: Marketers are often concerned about how much ISPs examine historical domain aspects like DNS records or hosting, hoping to avoid any negative associations from the past. For a general understanding of how to protect your domain, consider learning about email deliverability factors.
• Perception of risk: Some marketers find the idea of a dormant key being potentially compromised and used for impersonation to be unsettling, even if the likelihood is extremely low.
• DMARC monitoring for anomalies: Marketers using DMARC hope that its reports would catch any unexpected traffic or misuse of old keys, providing an alert system even if the email appears authenticated.
• Proactive cleanup: The general sentiment is that it's better to be safe than sorry and proactively delete unused DNS records for clarity and security. This aligns with overall best practices for maintaining sender reputation.

Key considerations

• Avoiding 'dirty fingerprints': Marketers are keen to ensure no lingering data points could potentially hurt their domain's standing or link them to a low-reputation history.
• DNS management complexity: The process of managing multiple DNS records, especially after changing ESPs, can be daunting for marketers who are not primarily technical.
• Focus on active sending: While concerned about past records, marketers should primarily focus on building a strong reputation with their current ESP and active sending infrastructure.
• Understanding authentication: A clear understanding of SPF, DKIM, and DMARC is crucial for marketers to confidently manage their email infrastructure.

Key opinions

• Passive records: DKIM records are inert unless mail is actively being sent from the associated ESP using that specific key. The published public key in DNS itself does not carry reputation; it's the signed email that contains the identifier.
• Reputation shift: When moving ESPs, spam filters (mailbox providers) gradually shift their reputation assessment to the new sending streams. While initial reputation might carry over due to DKIM consistency, the new sending behavior quickly becomes dominant. This also applies to considerations when changing ESPs and domains.
• Domain vs. IP reputation: High-tech mailbox providers increasingly focus on the reputation of the mail stream, primarily tied to the DKIM `d=` domain, rather than solely on the sending IP address. IP reputation primarily helps with initial access, but domain-level reputation is key for ongoing deliverability.
• Small security risk: A very remote risk exists that the private key corresponding to an old, still-published DKIM record could be compromised. If this happened, an attacker could send authenticated emails from your domain. However, DMARC reports could help reveal such unauthorized traffic, even if valid. You can learn more about DKIM replay attacks.

Key considerations

• DNS cleanliness: Experts recommend removing unused DKIM records for better DNS organization and simplified maintenance, rather than for a direct deliverability improvement.
• Key rotation implications: Regular DKIM key rotation is a good security practice to minimize the window of vulnerability for any given key. This ties into how DKIM selectors affect reputation and best practices.
• Monitoring authenticated traffic: Even if mail passes authentication, DMARC reports are crucial for identifying unexpected sources or types of authenticated traffic, helping to detect potential misuse of old keys or unauthorized sending.
• Focus on active authentication: Ensure your current sending infrastructure is properly authenticated with up-to-date SPF, DKIM, and DMARC records to maximize current deliverability.

Key findings

• Authentication importance: Documentation frequently highlights that email authentication (SPF, DKIM, DMARC) is essential for receiving mail servers to confirm the legitimacy of incoming emails. Without it, messages are prone to being flagged as spam or rejected. For more details on this, refer to authentication's impact on deliverability.
• Sender verification: DKIM, alongside SPF and DMARC, provides a mechanism for mail servers to verify that an email truly originates from the stated sender and has not been tampered with in transit. This verification is a key trust signal.
• Impact on deliverability: Properly implemented authentication contributes significantly to improved email deliverability, reducing the likelihood of messages landing in the spam folder. Conversely, missing or incorrect authentication can severely harm inbox placement. You can learn how SPF, DKIM, DMARC, and dedicated IPs affect deliverability.
• Domain reputation enhancement: Beyond just deliverability, DMARC (which relies on SPF and DKIM) helps enhance and protect domain reputation by providing policies for handling unauthenticated mail.

Key considerations

• Consistent authentication: Maintaining consistent and accurate authentication across all active sending platforms is paramount. Any inconsistency (e.g., sending some emails without DKIM) can raise red flags for ISPs.
• Active management: DNS records, including DKIM, should be actively managed and updated as sending infrastructure changes to avoid misconfigurations or stale entries.
• Security implications: Documentation often implies that unmanaged DNS records can contribute to a broader attack surface for phishing and spoofing, underscoring the need for proper key management.
• Monitoring and reporting: Leveraging DMARC reporting is consistently advised to gain visibility into email authentication results and detect any unauthorized sending from your domain, regardless of the DKIM key used.