What causes Gmail delays due to unusual sending rates and suspicious content?
Michael Ko
Co-founder & CEO, Suped
Published 1 Jun 2025
Updated 17 Aug 2025
7 min read
When you send emails to Gmail users, you might occasionally encounter delays or even outright blocks. These aren't random occurrences, but rather signals from Google's sophisticated spam and abuse prevention systems. Often, these issues stem from an unusual sending rate or suspicious content within your messages. Understanding why these delays happen is crucial for maintaining good email deliverability.
Gmail's systems are designed to protect its users from spam and malicious content. When they detect something out of the ordinary, whether it's a sudden surge in your email volume or specific elements within your email that raise red flags, they'll often defer the delivery. This deferral gives them time to perform additional checks and decide if the email is legitimate or potentially harmful.
The error messages you receive, such as 421-4.7.28 Gmail has detected an unusual rate of mail or 421-4.7.0 message is suspicious due to the nature of the content, provide clues about the root cause. It's often a combination of factors related to your sender reputation, email content, and sending patterns. Understanding these indicators is key to troubleshooting and improving your email deliverability, especially when sending to Gmail recipients.
Understanding Gmail's rate limiting
One of the most common reasons for Gmail delays is an unusual sending rate. This doesn't necessarily mean you're sending too many emails in total, but rather that your current sending volume or pattern deviates significantly from your historical sending behavior or what Gmail expects from your domain's reputation. A sudden spike in volume, particularly from a domain that hasn't sent much mail before, is a major red flag.
Gmail uses a variety of signals to build a reputation score for your sending domain and IP address. If you suddenly start sending a much higher volume of emails than usual, Google's systems might interpret this as suspicious activity, potentially indicating a compromised account or a spam campaign. This often results in a 421-4.7.28 deferral message.
These rate limits are dynamic and depend heavily on your historical sending patterns, sender reputation, and recipient engagement. Even with proper email authentication protocols in place, an unusual rate can trigger a review. It's a protective measure to ensure the integrity of their network and the inboxes of their users.
The problem with unusual rates
Gmail's systems are constantly evaluating incoming mail for signs of abuse. An 'unusual rate' indicates a deviation from your established sending patterns, which can flag your emails as potentially problematic. This isn't just about volume, but also about the consistency and quality of your sending behavior over time.
The primary goal is to prevent spam and phishing. If an email campaign drastically increases in volume without a corresponding positive reputation, Gmail assumes a risk. This often leads to SMTP errors like 421-4.7.28, indicating that the emails are temporarily delayed or rejected to protect users from spam.
The role of content and links
Beyond the sending rate, the actual content of your emails plays a significant role in deliverability. Gmail's filters are incredibly sophisticated and can detect suspicious elements within the email body, subject line, and even the links embedded in your message. If an email is flagged as suspicious, you might see a 421-4.7.0 message is suspicious due to the nature of the content and/or the links within.
Common culprits include overly spammy phrases, excessive use of all caps, too many exclamation marks, or suspicious-looking URLs. Attachments, especially certain file types, can also trigger alerts if they are uncommon or associated with malware. The way your HTML is structured can also be a factor, with overly complex or malformed HTML sometimes leading to issues.
Links within your emails are particularly scrutinized. If the links point to domains with poor reputations, or if they look like phishing attempts (e.g., deceptive URLs, suspicious redirects), your email is likely to be delayed or sent straight to the spam folder. It's not enough to just have clean content, the integrity of your links is equally important.
Safe content practices
Personalization: Address recipients by name and use relevant content to avoid generic spam triggers.
Clear subject lines: Be concise and accurately reflect email content, avoiding misleading or overly promotional language.
Reputable links: Ensure all URLs in your email lead to legitimate, well-regarded websites. Consider using HTTPS for all links.
Balanced text-to-image ratio: Avoid emails composed almost entirely of images, which can be a spam indicator.
Risky content practices
Spam trigger words: Words like 'free money,' 'guaranteed income,' or excessive use of financial terms can trigger filters.
Suspicious attachments: Avoid unusual file types or large attachments, especially if not explicitly expected by the recipient.
Misleading URLs: Links that don't match the displayed text, or lead to phishing sites, are critical red flags.
Broken HTML/CSS: Poorly coded emails can be harder for Gmail to parse and may lead to increased scrutiny or outright blocking.
Authentication and reputation fundamentals
A robust email authentication setup is foundational to good email deliverability. SPF, DKIM, and DMARC are crucial for proving that your emails are legitimate and that you are authorized to send from your domain. If these records are missing, improperly configured, or fail during verification, Gmail is far more likely to defer or reject your emails, regardless of your sending volume or content quality.
Gmail specifically mentions unusual rate of mail originating from your DKIM domain or SPF domain. This indicates that the authentication itself is tied to the rate limiting. If your authentication is weak or inconsistent, it signals to Gmail that your domain might be compromised or that you're attempting to spoof emails, leading to delays and potential blacklisting (or blocklisting) of your sending IP or domain. Checking your domain's reputation with Google Postmaster Tools is a critical step.
Sender reputation, which encompasses both your IP and domain reputation, is continuously assessed by Gmail. Factors like spam complaints, bounce rates, and engagement metrics (opens, clicks) all contribute to this score. A poor reputation (or lack of established reputation for new senders) combined with an unusual sending rate or suspicious content is a recipe for delays and inbox placement issues. Maintaining a clean sending history and engaged audience is paramount.
Builds on SPF and DKIM, telling recipient servers how to handle emails that fail authentication.
Strongly recommended by Gmail for bulk senders to enhance trust and deliverability.
Strategies for recovery and prevention
If you're experiencing delays due to unusual sending rates or suspicious content, the first step is to identify the exact cause. Use tools like Google Postmaster Tools to monitor your sender reputation, spam rates, and delivery errors. This data is invaluable for understanding how Gmail perceives your sending practices.
To recover and prevent future delays, you need a multi-faceted approach. This includes a careful warming-up strategy for new domains or significant increases in volume, ensuring your email lists are clean and engaged, and reviewing your content for anything that might trigger spam filters. Consistent sending with high engagement helps build a positive reputation over time.
Best practices for avoiding delays
Warm-up your sending IPs and domains: Gradually increase your sending volume to new domains or IP addresses. This builds trust with Gmail and other mailbox providers.
Maintain high engagement rates: Send relevant content to an engaged audience to encourage opens and clicks, and minimize unsubscribes and spam complaints.
Regularly clean your email lists: Remove inactive or invalid addresses to reduce bounce rates and avoid spam traps. This also helps with Gmail bounce issues.
Monitor your deliverability: Use monitoring tools to track your inbox placement, spam complaint rates, and blacklist (blocklist) status.
Views from the trenches
Best practices
Maintain consistent sending volumes to avoid sudden spikes that trigger rate limits.
Segment your audience and personalize content to improve engagement and reduce spam complaints.
Regularly audit email content and links for suspicious patterns or known spam triggers.
Prioritize list hygiene to remove unengaged subscribers and prevent bounces.
Common pitfalls
Abruptly increasing sending volume without a proper warm-up strategy.
Sending emails with generic, overly promotional, or poorly formatted content.
Using purchased or old email lists that lead to high bounce rates and spam traps.
Ignoring authentication failures (SPF, DKIM, DMARC) or not having them configured.
Expert tips
If your ESP uses shared tracking domains, ensure their overall reputation is good.
Consider white-labeling your tracking domains to use your own domain's reputation.
When troubleshooting, differentiate between issues from your sending platform and customer-specific behavior.
Address 'suspicious content' warnings by thoroughly reviewing email body, subject, and all embedded links.
Marketer view
Marketer from Email Geeks says an unusual sending rate typically means exactly what it states, an atypical email flow.
2024-07-23 - Email Geeks
Marketer view
Marketer from Email Geeks says checking authentication protocols and sender domain reputation is a good first step. They also recommend inspecting content and HTML, verifying SSL certificates for links and tracking, optimizing campaign cadence, and using Google Postmaster Tools. A reduced sending volume followed by a ramp-up strategy and KPI monitoring is also advised.
2024-07-23 - Email Geeks
Ensuring smooth Gmail delivery
Dealing with Gmail delays due to unusual sending rates and suspicious content requires a proactive and informed approach. It's a clear signal from Google that something in your email program needs attention. By focusing on consistent sending patterns, ensuring strong authentication, and maintaining high-quality, relevant content, you can build and preserve the trust necessary for reliable deliverability.
Remember, Gmail's primary goal is to protect its users. By aligning your sending practices with their guidelines, you not only avoid delays and spam placements but also contribute to a healthier email ecosystem overall. Regular monitoring and quick action when issues arise are your best defense against these common deliverability challenges.
