Why is a new IP address showing up in my GPT dashboard with SPF failing but DKIM passing?

Key findings

• Unexpected IP source: A new IP address, not owned by the ESP (Email Service Provider), appeared in GPT, indicating significant mail volume.
• Authentication status: Messages from this IP passed DKIM and DMARC, yet failed SPF. This suggests the DKIM signature held by the ESP was correctly applied, even though the sending IP was unauthorized by SPF.
• ESP non-involvement: The ESP confirmed the IP was not theirs and declined responsibility, leaving the sender to investigate.
• SPF alignment: The SPF domain in question was identified as a bounce subdomain belonging to the sender, with a softfail result. This indicates a misalignment where the Return-Path domain did not align with the From domain for DMARC validation purposes.
• Volume anomaly: The high volume of mail from this unexpected IP, enough to register in GPT, raised questions about widespread forwarding or other unusual mail routing.

Key considerations

• Investigate DMARC reports: Thoroughly analyze raw DMARC reports to verify the reported IP, its volume, and authentication results. These reports provide granular data that can confirm or dispute what is seen in aggregated tools like GPT. Understanding and troubleshooting DMARC reports is key.
• Mail forwarding: Consider the possibility of mass mail forwarding by a recipient organization. While forwarding typically breaks SPF, the consistent DKIM pass (especially if the DKIM signature isn't broken) could indicate the original message was legitimate but rerouted through an unapproved path. However, high volume forwarding through a single IP is unusual.
• Potential compromise: If the ESP's DKIM key is being used by an unauthorized IP, it could suggest a compromise or exploit of the ESP's infrastructure or the domain's DNS records, particularly the public DKIM key. Regenerating DKIM keys and securing DNS is a critical step.
• Google Postmaster Tools accuracy: Be aware that GPT's data, while valuable, may not always be perfectly accurate, especially with complex mail flows or forwarding scenarios. Google itself notes it makes its best effort but doesn't guarantee accuracy in all cases due to forwarding.
• Domain reputation impact: Even with DKIM passing, SPF failures can negatively impact domain reputation and inbox placement. Continuous SPF failures, especially from unexpected IPs, can lead to messages being blocked or routed to the spam folder. Knowing what to do with unexpected IPs in GPT is essential.

Key opinions

• ESP dismissal: Many marketers report that when an issue arises from an IP not explicitly managed by their ESP, the provider often quickly disclaims responsibility, leaving the sender to investigate independently.
• Search for explanations: Marketers frequently seek speculative advice from peers when direct answers from providers are unavailable, as every potential clue can lead to a solution.
• Forwarding suspicions: The most common initial thought for SPF failures with DKIM passes is mail forwarding, where an email is re-transmitted through another server that isn't included in the original SPF record.
• Volume concern: A key point of confusion is when forwarded mail volume is so high it consistently appears in Google Postmaster Tools, suggesting more than casual individual forwarding.
• Reputation worries: Even if DMARC passes due to DKIM, marketers worry that consistent SPF failures from unexpected sources could still negatively impact their domain's sending reputation and potentially lead to blacklisting or blocklisting.

Key considerations

• Deep dive into DMARC reports: Marketers should consistently check their DMARC aggregate reports (RUAs) for detailed insights into authentication failures, including the source IPs and their associated volumes. This data is critical for understanding why your DMARC success rate might be dropping.
• Investigate DKIM selectors: Identifying the DKIM selector used by the unexpected IP can help pinpoint which signing authority is involved, even if the IP isn't directly controlled by the ESP. This might uncover a delegated sending service or a compromised subdomain.
• Confirm IP ownership: While a reverse IP lookup can suggest ownership (e.g., Comcast), further investigation through WHOIS data or direct contact with the perceived owner may be necessary if the volume is substantial.
• Review SPF records: Ensure your SPF record is accurately configured and includes all legitimate sending sources. A misconfigured SPF or an unauthorized IP not listed will lead to SPF failures.
• Consider security audits: If unauthorized sending is suspected, conduct a security audit of your ESP account and any systems authorized to send emails on your behalf to rule out a compromise or exploit.

Key opinions

• Compromise risk: Experts often suggest the possibility of an ESP compromise or exploit if an unauthorized IP is signing messages with the client's DKIM key, as this implies access to the private signing key.
• DKIM key management: The advice to regenerate DKIM keys or delete the public key from DNS points to a critical security measure to stop unauthorized signing, emphasizing that DKIM's strength relies on the secrecy of the private key.
• Forwarding behavior: While forwarding commonly causes SPF failures, experts clarify that DKIM is often preserved through forwarding, which aligns with the observed SPF fail/DKIM pass scenario.
• DMARC alignment rules: Experts highlight that SPF alignment is specifically a DMARC concept, controlled by the aspf= tag in the DMARC record. This explains why a bounce subdomain would cause SPF alignment failure under a strict policy.
• GPT data reliability: Experts acknowledge that Google Postmaster Tools, while useful, may not always provide perfectly accurate or complete data, particularly in complex scenarios involving forwarding, and it might even exhibit reporting bugs.

Key considerations

• Verify DMARC report volume: Always cross-reference GPT data with raw DMARC aggregate reports to confirm the reported IP and its associated email volume. This is essential for distinguishing significant traffic from minor anomalies, as explored in DMARC success rate fluctuations.
• Examine the DMARC reporter: Check if the DMARC reports originate solely from Google or from various reporters (e.g., Verizon, Comcast). A single dominant reporter suggests an issue with that specific recipient organization's mail routing or forwarding setup.
• Understand SPF alignment: Be aware of your DMARC policy's SPF alignment setting (relaxed 'r' or strict 's'). A strict 's' policy will cause SPF alignment failure if the Return-Path domain doesn't exactly match the From domain, common with bounce subdomains.
• Security implications: If an ESP or other provider is indeed compromised, it represents a significant security breach. It's crucial to address this swiftly to prevent abuse of your domain's sending authority and potential phishing or spam activity.
• Proactive monitoring: Regularly monitor DMARC reports and Google Postmaster Tools for any anomalies, as early detection can mitigate severe deliverability and reputation damage, especially regarding fluctuations in SPF authentication.

Key findings

• SPF validation: SPF (Sender Policy Framework) verifies the sending IP address against a list of authorized IPs published in the sender's DNS. If the IP is not listed, SPF fails.
• DKIM validation: DKIM (DomainKeys Identified Mail) uses cryptographic signatures to ensure that an email has not been tampered with in transit and was sent by an authorized party. A DKIM pass indicates the signature is valid and the message integrity is intact.
• DMARC enforcement: DMARC (Domain-based Message Authentication, Reporting & Conformance) leverages both SPF and DKIM. For DMARC to pass, at least one of these must pass and align with the From domain. This explains how DMARC can pass even with SPF failing if DKIM is aligned.
• SPF alignment types: DMARC defines strict (Return-Path must exactly match From domain) and relaxed (Return-Path can be a subdomain of the From domain) SPF alignment modes, specified by the aspf tag in the DMARC record.
• Mail forwarding impact: Mail forwarding often alters the Return-Path (or MAIL FROM) address, causing SPF to fail authentication from the forwarding server's IP. However, DKIM is more resilient to forwarding unless the message content itself is modified, as explained in articles about how DMARC prevents email spoofing.

Key considerations

• RFC 7208 (SPF): Refers to the original standard for SPF. SPF checks if the sending IP is authorized by the domain listed in the Return-Path (or MFROM) address. If the IP is not in the SPF record, authentication will fail.
• RFC 6376 (DKIM): Details the DKIM protocol, which uses a pair of cryptographic keys: a private key to sign the email and a public key published in DNS for verification. As long as the message headers/body included in the signature aren't modified, DKIM should pass even after forwarding.
• RFC 7489 (DMARC): Outlines DMARC's role in instructing receiving mail servers on how to handle emails based on SPF and DKIM authentication and alignment. It also specifies the XML format for aggregate (RUA) reports, which provide granular data on authentication results, including source IPs and message volumes. For more details, consult an engineer's guide to email delivery.
• Google Postmaster Tools limitations: Official Google documentation indicates that GPT data is aggregated and may not perfectly reflect individual message authentication, particularly due to forwarding. It's designed to provide insights into patterns and trends, not necessarily real-time granular data for every message. For deeper analysis, the Ultimate Guide to Google Postmaster Tools is invaluable.