March 22 2024

Gmail Introduces New Spam-Related Reject Messages: What You Need to Know

Understand Gmail's spam-related rejection rules and essential strategies to optimize sending and improve the deliverability of your emails.


Gmail recently introduced a series of new decline/defer messages. It's critical that you familiarize yourself with the following Gmail rejection messages to better understand how to manage and prevent email delivery issues:

  • “421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit to review our Bulk Email Senders Guidelines. – gsmtp”
  • “421 4.7.28 Gmail has detected an unusual rate of unsolicited mail containing one of your URL domains. To protect our users from spam, mail with the URL has been temporarily limited. Please visit to review our Bulk Email Senders Guidelines. – gsmtp”
  • “421 4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your SPF domain. To protect our users from spam, mail sent from your domain has been temporarily rate limited. Please visit to review our Bulk Email Senders Guidelines. – gsmtp”
  • “421-4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your DKIM domain. To protect our users from spam, mail sent from your domain has been temporarily rate limited. Please visit to review our Bulk Email Senders Guidelines. – gsmtp”

The first message is not completely new; although it is not commonly encountered, it has been around for some time. However, in recent years, Google has progressively increased the number of cases in which it applies this type of refusal or deferral to the delivery of an email. With the introduction of new policies and specific messages, Google is clarifying the specific reasons why certain messages are not accepted.

Meaning of the New Rejection/Deferral Messages

URL domain: one or more domains present in the links in your emails have a negative reputation linked to spam. Gmail doesn't specify which link is problematic, it could be a shared link (such as an ESP's unsubscribe link) or a website chosen to be included in the email.

SPF domain: this domain appears in the return address or MFROM address, an email address in a hidden header called “Return-Path”. This could be a custom domain configured on the email sending platform, or a shared domain used by the platform.

DKIM domain: refers to the domain used in the DomainKeys Identified Mail (DKIM) authentication header, which contains a cryptographic signature to verify that a given domain is the sender or responsible for an email message. A message can have more than one DKIM signature, but it is more common for it to have just one or two (your domain AND the platform's default DKIM domain).

Using shared resources can be a problem. If a sending platform (ESP, CRM, etc.) can only use a default or shared domain for these settings, a block against one domain can affect many customers, even unrelated ones. A block against a shared domain used by all customers will negatively impact delivery ability for ALL customers of that ESP. If an email sending platform doesn't adequately monitor for malicious activity, you expose yourself to risk.

Avoiding shared domains does not guarantee immunity from problems, but it prevents you from being involved in platform-related issues. If your SPF domain, DKIM domain, and links in emails are all associated with your domain, any blocks against these domains affect you directly and may indicate spam-related delivery issues that require your investigation and resolution. It's important to note that Gmail is becoming increasingly strict in dealing with practices that were previously less likely to cause crashes. As a result, the “I never got blocked for this in the past” argument no longer applies. Gmail, like almost all Internet service providers and mail management platforms, is improving its ability to reject junk mail. The main question therefore becomes: why is an email considered unwanted and how to solve the problem? These are the first questions to ask yourself to identify the cause of the problem and plan an appropriate solution.

Further information: Speed ​​Limitation and Response 421

When Gmail responds to an email delivery attempt with a rejection message, it is not categorically rejecting all messages. This is a temporary deferral, as indicated by the numeric code 421, which begins with 4 (deliverability experts call this a 4xx response). The full “421” code provides no particular insight, simply meaning “temporary deferral,” but the text associated with that decline makes it clear that the issue is spam-related and that Gmail is limiting the speed at which email attempts are sent. This means that access is made more restrictive, allowing only a limited number of messages to pass through – and this number could be as low as zero. The limitation concerns the speed with which the sending mail server is allowed to send messages, thus reducing the number of messages that can be delivered.

Since the messages are "deferred" and not immediately rejected, they remain in the email queue of the sending server (MTA). This server will periodically retry these messages, but will generally give up after a longer period of time. When the sending platform gives up, it will record the message and the recipient as undeliverable. As a result, these spam issues lead to delays in delivery, logging, and highlighting of undelivered messages. Therefore, do not assume that, in the absence of rebounds, everything is in order. If you or your sending platform has the ability to monitor mail server queues in real time, now is the time to check what's happening.

Strategies to Avoid Spam-Related Rejections

To effectively navigate the evolving landscape of Gmail spam management, here are some strategies:

  1. Checking and Cleaning Lists: Keep your email distribution lists clean and up to date. Regularly remove inactive or bouncing addresses.
  2. Optimize Email Authentication: Make sure your emails are authenticated correctly using SPF, DKIM and DMARC. This not only improves deliverability but also helps prevent rejection for spam-related reasons.
  3. Monitor Your Domain Reputation: Use tools to monitor your domain reputation and sending IP addresses. A bad reputation can be a sign of problems that need to be resolved to avoid Gmail throttling.
  4. Engagement and Relevance of Content: Sending relevant and interesting content can improve user engagement, reducing the risk of your emails being reported as spam.
  5. Comply with the Guidelines for Sending Mass Emails: Familiarize yourself with Gmail's guidelines for mass emailing and make sure you follow them to avoid rejections and throttling.

By adopting these practices, you will not only improve your ability to avoid Gmail rejections and throttling, but you can also significantly improve the deliverability of your email campaigns across all Internet Service Providers (ISPs).

Here are some additional strategies and final considerations to further optimize the management of your email communications.

Personalization and Segmentation

Email personalization and segmentation not only increases recipient engagement, but also reduces the risk of your messages being perceived as unsolicited or spam. Sending the right content to the right audience at the right time is crucial to maintaining a good sender reputation and ensuring your emails are received positively.

Pay attention to links and images

Please carefully check the links and images included in your emails. Broken links or excessive use of images (especially with little or no text) can trigger spam filters. Make sure all links are valid and relevant to the content of the email and that the use of images is balanced with relevant text.

Sending Frequency

Monitoring and optimizing the sending frequency of your email campaigns is essential. Sending too many messages in a short period of time can not only bore or annoy your recipients, but can also trigger rate limiting systems like those implemented by Gmail. Find the right balance in sending frequency that keeps your audience interested without overwhelming them.

Feedback and Monitoring

Use recipient feedback to guide your email strategies. This includes tracking open rates, clicks, unsubscribes and spam reports. This data can provide valuable insights into what works and what doesn't, allowing you to adjust your strategies accordingly.

Education and Transparency with Recipients

Make sure your recipients know why they're receiving your emails and how they can easily manage their subscription preferences or unsubscribe if they choose. Transparency and respecting recipient preferences are not only good ethical practices, but also help reduce the risk of being reported as spam.


Navigating the challenges posed by modern spam filters requires a holistic approach that combines technology, content strategy and email marketing best practices. By maintaining a strong sender reputation, adhering to ISP guidelines, and continually optimizing your sending practices, you can ensure your emails reach the intended inboxes, avoiding spam traps, and maximizing recipient engagement.

Remember that the goal isn't just to avoid rejections from Gmail or other providers, but to build valuable relationships with your recipients by delivering content that is relevant, engaging, and desired. With an unwavering commitment to excellence in email marketing practices, you can successfully navigate the increasingly complex landscape of email deliverability.

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