What causes Gmail DKIM domain rate limiting errors and how are they related to SPF?

Key findings

• Authentication issues: The 421-4.7.28 error explicitly points to an unusual rate of unsolicited mail originating from your authenticated DKIM or SPF domain, signaling a potential problem with how your emails are perceived.
• Rate limiting: This is a temporary measure by Gmail to prevent spam, affecting a small number of senders initially. However, it serves as an early warning.
• Interconnectedness of SPF and DKIM: Observations suggest that issues with SPF can directly influence DKIM-related bounces, indicating that Gmail's filtering mechanisms evaluate these authentication protocols in tandem.
• Empty domain brackets: The appearance of empty brackets like [ 15] in the error message is unusual and might be a system anomaly or a placeholder for internal identifiers.
• Compliance emphasis: The error message directs senders to review Google's bulk email sender guidelines, underscoring the importance of adherence to their evolving policies, especially in light of recent and upcoming changes.

Key considerations

• Review guidelines: Familiarize yourself with and adhere to Google's Bulk Email Senders Guidelines, as compliance is crucial for avoiding deliverability issues.
• Strengthen authentication: Ensure your SPF, DKIM, and DMARC configurations are robust and correctly aligned. A simple guide to DMARC, SPF, and DKIM can help you verify your setup.
• Monitor sending practices: Proactively monitor your email program for any sudden increases in volume, spam complaints, or changes in engagement that could trigger rate limits. Utilize tools like Google Postmaster Tools to track your sender reputation.
• Address alignment: Focus on the alignment of your RFC5322 From: header with your authenticated SPF and DKIM domains to ensure proper email authentication.

Key opinions

• New error pattern: Many marketers are seeing this specific 4.7.28 error for the first time, especially with the [ 15] anomaly.
• Mild impact (for now): The rate limiting is perceived as mild and affecting a limited number of senders, suggesting it might be a gradual rollout or a targeted measure.
• Anticipating 2024 changes: There's a strong belief that these errors are Google's ongoing work in progress for the February 2024 sender requirements.
• SPF and DKIM correlation: Some marketers have noted that addressing issues with SPF-aligned sending (e.g., blocking spammy SPF senders) can resolve DKIM-related bounces, indicating a strong interplay between the two.
• Authentication is key: Marketers universally agree that robust SPF, DKIM, and DMARC setup is crucial to prevent Gmail from blocking emails or applying rate limits.

Key considerations

• Monitor bounce messages: Actively track and categorize your bounce messages for new error codes or patterns, as this can provide early warnings of deliverability issues. Understanding what causes a sudden spike in bounce rates is essential.
• Verify authentication: Regularly check the status of your SPF and DKIM authentication to ensure they are correctly set up and aligned. Issues with one can affect the perception of your overall domain reputation.
• Prepare for policy changes: Be proactive in adjusting your sending practices to comply with Google's updated bulk sender policies. This includes maintaining low spam rates and ensuring authenticated sending for all mail.

Key opinions

• System bug or aligned domain reaction: The empty brackets in the error message are an unusual artifact, potentially a bug or a specific identifier for an aligned domain triggering the rate limit.
• SPF impact on DKIM: Experts note that blocking problematic SPF senders can resolve subsequent DKIM bounces, suggesting that a domain's SPF reputation directly influences its DKIM deliverability and vice-versa.
• Spam rate correlation: These rate limits are primarily triggered by an unusual rate of unsolicited mail, directly tied to the sender's perceived spam rate and overall sending quality.
• Precursor to policy changes: Many view these transient errors as Google's initial steps towards enforcing their stricter bulk sender guidelines for 2024, acting as an early warning system.
• Importance of full authentication: The occurrence of both SPF and DKIM-related rate limits underscores the necessity of having both protocols correctly configured and aligned for optimal deliverability.

Key considerations

• Implement DMARC: Leverage DMARC monitoring to gain comprehensive visibility into your authentication failures for SPF and DKIM, which directly impact your domain reputation.
• Align headers correctly: Ensure your RFC5322 From: header is properly aligned with your authenticated SPF or DKIM organizational domain. This is a critical factor in avoiding rate limiting and soft bounces due to authentication failures.
• Proactive sending adjustments: Continuously adjust sending practices to comply with Google's evolving guidelines. This includes managing list hygiene and ensuring content relevance to minimize unsolicited mail complaints.
• Address problematic senders: If a specific SPF sender is causing these rate limits, consider temporary measures like blocking them to prevent a ripple effect on your DKIM reputation.

Key findings

• Unsolicited mail flag: The core of the Gmail error is the detection of an "unusual rate of unsolicited mail" from an authenticated DKIM or SPF domain, directly linking content and sender reputation to authentication.
• Guideline enforcement: The error explicitly directs senders to review bulk email sender guidelines, indicating that the rate limit is a consequence of non-compliance rather than a random technical glitch.
• Header alignment crucial: Documentation confirms that lack of alignment between the RFC5322 From: header and authenticated SPF or DKIM is a direct cause for rate limiting and other bounce errors.
• Temporary errors as signals: Bounce codes like 421 4.7.32 serve as temporary warnings, providing senders an opportunity to identify and resolve issues before facing permanent blocklists (or blocklists).
• Comprehensive authentication: Authentication basics across documentation consistently highlight that proper SPF, DKIM, and DMARC are fundamental requirements to avoid blocking and ensure mail delivery.

Key considerations

• Strictly adhere to guidelines: Ensure your sending practices align precisely with Google's updated Bulk Email Senders Guidelines, focusing on authentication, spam rates, and explicit consent.
• Verify SPF and DKIM setup: Regularly confirm that your SPF and DKIM records are correctly configured and aligned with your sending domain to pass authentication checks.
• Utilize DMARC for visibility: Implement DMARC monitoring to receive aggregated reports on your email authentication status, helping identify potential failures before they lead to rate limits or blocking.
• Address spam rate: A high spam complaint rate (above 0.3% for Gmail) is a critical indicator of unsolicited mail and will trigger rate limits. Focus on list hygiene and relevant content to keep this rate low.