What causes Gmail DKIM domain rate limiting errors and how are they related to SPF?
Michael Ko
Co-founder & CEO, Suped
Published 24 May 2025
Updated 10 Oct 2025
7 min read
Receiving a Gmail DKIM domain rate limiting error can be a frustrating experience. It tells you that your emails are being temporarily delayed or blocked by Gmail because an unusual rate of unsolicited mail is originating from your domain. While the error specifically mentions DKIM, the underlying causes are often more complex and frequently tied to your overall sender reputation and Sender Policy Framework (SPF) configuration.
It's easy to assume the problem lies solely with your DKIM setup, but in many cases, issues with SPF can indirectly contribute to or even trigger these DKIM-related rate limits. Gmail, like other major mailbox providers, evaluates a multitude of signals to assess the trustworthiness of incoming mail. A weakness in one authentication pillar, such as SPF, can impact the perceived legitimacy of your entire sending operation, even affecting how DKIM is viewed.
Understanding this intricate interplay is crucial for maintaining strong email deliverability and avoiding unnecessary disruptions. By delving into the specific causes of these rate limits and their connection to SPF, you can diagnose problems more effectively and implement robust preventative measures.
When Gmail detects an unusual rate of unsolicited mail from a DKIM-signed domain, it issues a 421-4.7.28 error, temporarily rate-limiting messages from that domain. This isn't a permanent block, but a cautionary measure designed to protect recipients from potential spam or abusive sending practices. The DKIM domain mentioned in the error refers to the domain that is cryptographically signing your emails.
Typical Gmail rate limiting error message
Error message
421-4.7.28 Gmail has detected an unusual rate of unsolicited mail originating\r\n421-4.7.28 from your DKIM domain [ 15]. To protect our users from spam,\r\n421-4.7.28 mail sent from your domain has been temporarily rate limited. Please\r\n421-4.7.28 visit\r\n421-4.7.28 https://support.google.com/mail/?p=UnsolicitedRateLimitError to\r\n421 4.7.28 review our Bulk Email Senders Guidelines.
This message indicates that your sending practices or reputation have triggered a spam-prevention mechanism. The empty brackets[ 15] might suggest a bug or an evolving detection method from Gmail, making troubleshooting slightly more challenging initially.
Gmail's primary goal is to protect its users from unwanted messages. This means that any activity, even if legitimate, that resembles spam patterns, can lead to rate limiting. Factors like sudden spikes in sending volume, low engagement rates, high complaint rates, or consistent authentication failures can all contribute to a tarnished sender reputation, which in turn triggers these types of blocks. While DKIM is explicitly named, it often acts as a visible symptom rather than the sole root cause.
The foundation: how SPF protects your domain
SPF (Sender Policy Framework) is a crucial email authentication method that helps prevent spammers from sending messages on behalf of your domain. It allows domain owners to publish a list of authorized sending IP addresses in their DNS records. When an email server receives a message, it checks the SPF record of the sender's domain to verify if the sending server is indeed allowed to send mail for that domain.
An SPF record is a TXT record that lives in your domain's DNS. While essential, SPF records have limitations, such as a maximum of 10 DNS lookups. Exceeding this limit or having broken SPF records can lead to validation errors, weakening your email authentication posture.
Common SPF failures, such as syntax errors or an outdated record that doesn't include all sending sources, can significantly harm your deliverability. When SPF validation fails, recipient mail servers become suspicious, potentially leading to emails being flagged as spam or soft bounced. If you're encountering Gmail SPF error messages, addressing them quickly is vital.
The intricate relationship between SPF, DKIM, and rate limits
While SPF and DKIM are distinct authentication protocols, they often work in tandem, especially when DMARC is implemented. DMARC relies on the successful authentication of either SPF or DKIM, and crucially, on their alignment with the From: header domain. A failure in SPF can trigger a cascade of trust issues, even if your DKIM record is technically valid.
For example, when emails are forwarded to Gmail, the forwarding server often alters the email, causing SPF to fail. Although DKIM might remain intact, the overall authentication picture looks weaker to Gmail. This can lead to your emails being seen as suspicious, contributing to a lower sender reputation and potentially triggering a DKIM domain rate limit even if the DKIM signature itself is valid. SPF failures can lead to unexpected rate limiting issues.
Scenario: SPF failure
Impact on authentication: When SPF fails, it signals to recipient servers that the email might not be from an authorized source.
DMARC alignment issues: A failed SPF can prevent DMARC from passing, even if DKIM is valid, affecting overall trust.
Lower sender reputation: Repeated SPF failures degrade your domain's reputation with mailbox providers.
Consequences for DKIM
Gmail may lose trust in the sending domain's overall authentication, even if DKIM technically passes. This reduced trust can manifest as DKIM rate limits, indicating that the authentication ecosystem is not entirely robust.
Scenario: DKIM rate limit
Observed symptom: Email messages are temporarily blocked or significantly delayed by Gmail's servers.
Underlying causes
Poor sender reputation: A history of spam complaints or low engagement affects how Gmail perceives your domain.
Suspicious sending patterns: Sudden, unexplained spikes in email volume can trigger automated spam filters.
SPF issues contributing to lower trust: Even if not directly failing DKIM, SPF problems weaken overall domain trustworthiness.
Ultimately, Gmail's system assesses your entire sending ecosystem. All authentication protocols, including SPF, DKIM, and DMARC, must be robust and consistently pass to build and maintain a strong domain reputation. Google's guidelines explicitly recommend setting up SPF, DKIM, and DMARC for optimal delivery. You can review Google's email sender guidelines for more comprehensive details.
Strategies for troubleshooting and preventing future issues
The first and most critical step in addressing any email deliverability issue, including Gmail DKIM rate limiting, is to gain visibility into your email authentication status. Tools like Suped's DMARC monitoring provide aggregated DMARC reports that show you which emails are passing or failing SPF and DKIM, and why. This granular data is invaluable for diagnosing problems.
Ensure that your SPF, DKIM, and DMARC records are all correctly configured and up-to-date. Misconfigurations are a common source of authentication failures. If you're encountering Gmail SPF/DKIM issues, checking the authentication results in your email headers can provide immediate insights.
Issue
Authentication Affected
Solution
SPF softfail
SPF
Check your SPF record syntax and included domains, ensure all IPs are listed.
Beyond technical configurations, regularly reviewing your sending practices, maintaining clean and engaged mailing lists, and ensuring consistent sending volumes are crucial for maintaining a healthy sender reputation. A sudden change in any of these factors can trigger rate limiting. Proactive monitoring helps you catch and resolve issues before they escalate.
Views from the trenches
Best practices
Keep SPF records concise, ensuring they stay under the 10-lookup limit for efficiency.
Consistently monitor DMARC reports to quickly identify authentication failures and rate limits.
Implement separate sending domains for different email types to protect your reputation.
Ensure all valid sending sources are correctly authorized in SPF and DKIM.
Validate your DNS records frequently to prevent any unintended authentication breaks.
Common pitfalls
Ignoring initial DKIM rate limit warnings, which are often signals of broader reputation issues.
Overlooking how SPF failures, especially with forwarded emails, can degrade DKIM trust.
Delaying DMARC implementation, missing crucial insights into your email authentication status.
Not regularly auditing email sending practices for unusual volume or content spikes.
Expert tips
Leverage a reliable DMARC monitoring solution like Suped for comprehensive authentication reports.
Thoroughly inspect email headers for SPF, DKIM, and DMARC results when diagnosing issues.
A sudden spike in transient DKIM bounces often suggests underlying SPF or reputation problems.
Isolate problematic sending streams to prevent a single campaign from impacting your entire domain.
Regularly check for any external blacklistings (blocklists) that might affect domain reputation.
Expert view
Expert from Email Geeks says: It looks like the empty DKIM domain bracket in Gmail's rate limiting message might be a bug, but the rate limits themselves are likely a reaction to problematic aligned domains.
Nov 23, 2023 - Email Geeks
Marketer view
Marketer from Email Geeks says: I've seen similar transient DKIM [ 15] errors, indicating that this might be a widespread issue or a new signal being evaluated by Gmail.
Nov 23, 2023 - Email Geeks
Safeguarding your sender reputation
Gmail DKIM domain rate limiting errors are more than just a DKIM configuration problem. They are a clear signal that Gmail's systems perceive a risk associated with your sending domain, often stemming from broader issues related to your sender reputation and other authentication mechanisms like SPF. A well-maintained SPF record, alongside DKIM and DMARC, is fundamental to establishing and preserving trust with mailbox providers.
By actively monitoring your email authentication, promptly addressing any SPF or DKIM failures, and adhering to best sending practices, you can prevent these rate limiting errors and ensure your legitimate emails reach the inbox consistently. Proactive management of your authentication protocols is not just about compliance, it's about safeguarding your email program's effectiveness.
