What does 'rate limit exceeded' mean in email delivery, and how do I troubleshoot it?
Michael Ko
Co-founder & CEO, Suped
Published 3 Aug 2025
Updated 19 Aug 2025
7 min read
Encountering a 'rate limit exceeded' message in your email delivery reports can be a concerning issue for any sender. It typically means that the recipient's mail server or your own sending system has temporarily blocked your emails because you've sent too many messages within a specified period. This isn't necessarily a permanent block, but rather a temporary deferral or rejection designed to protect systems from overload, potential spam, or abuse.
Mailbox providers, like Gmail or Microsoft (Outlook/Hotmail), set these limits to maintain service stability and prevent spammers from flooding their users' inboxes. When you hit a rate limit, the server will often respond with a 4XX temporary error code, indicating that you should try sending the message again later. However, repeated violations can lead to more severe consequences, such as longer deferrals or even permanent blocklisting (or blacklisting) of your sending IP address or domain.
Understanding why these limits are in place and how to react to them is crucial for maintaining good email deliverability. It's a signal that something in your sending pattern or recipient engagement needs attention. Ignoring these warnings can lead to a significant drop in your inbox placement rates.
Why do email providers impose rate limits?
Email service providers (ESPs) and internet service providers (ISPs) implement rate limits for several critical reasons. Primarily, it's a defensive measure against malicious activity and resource exhaustion. Without these limits, a single bad actor could flood millions of inboxes, degrade server performance, or launch denial-of-service (DoS) attacks.
These limits also play a vital role in reputation management. By slowing down suspicious sending patterns, ISPs can better assess the legitimacy of the sender. For instance, a sudden, massive increase in sending volume from a new or low-reputation domain might trigger a rate limit as a precautionary measure. This gives the ISP time to analyze the traffic and sender behavior, deciding whether to accept the emails or mark them as potential spam. You can learn more about how Gmail delivery rate limits are affected by sender reputation.
Causes of rate limiting
High volume sending: Attempting to send too many emails to a specific domain or across the internet in a short period, especially if your sending history does not support such a volume.
Poor sender reputation: A history of high bounce rates, spam complaints, or sending to invalid addresses can lead to ISPs throttling your mail. Being on a blocklist (or blacklist) will severely impact this.
Sending to non-existent users: A large number of hard bounces signals a poorly maintained mailing list or a spamming attempt, triggering rate limits.
Spam trap hits: Sending to spam traps can immediately trigger severe rate limits and blocklisting.
Moreover, some providers have explicit limits based on their infrastructure capacity or specific policies, regardless of sender reputation. These can be per-hour, per-day, or even per-connection limits. For instance, Gmail has documented sending limits for its users, which, if exceeded, will result in rate limit errors. Understanding these underlying reasons helps in effective troubleshooting and proactive management.
Diagnosing 'rate limit exceeded' errors
When you encounter a 'rate limit exceeded' message, the first step is to dig into your email logs. Your email service provider (ESP) or mail server should provide detailed bounce and deferral logs that contain the exact error messages from the receiving server. These messages often include specific error codes or phrases that can pinpoint the exact reason for the rate limiting.
Look for messages like:
421 Rate limit reached: A common temporary error indicating too many emails in too short a time.
450 4.7.1 Rate limiting for excessive data use: Often seen with providers like MailChannels, indicating high volume or unusual traffic.
429 Too many requests: General API rate limit error, applicable to email sending APIs.
550 5.7.233 Your message can't be sent because your tenant exceeded its daily limit for sending email: A specific error often from GoDaddy or Microsoft 365, indicating an account-level sending limit.
Additionally, monitor your sending volume over time. If you see a sudden spike in 'rate limit exceeded' bounces or deferrals, correlate it with recent changes in your sending patterns, list acquisition, or campaign launches. Unusual sending activity, like an unexpected surge in volume, is a common trigger for ISPs to apply rate limits, even if your overall reputation is good. Microsoft's documentation confirms that rate limiting is a server policy to protect infrastructure.
Troubleshooting and mitigation
Once you've identified the specific error messages and patterns, you can begin troubleshooting. The approach depends on whether the rate limit is due to reputation, volume, or specific recipient issues.
Actionable solutions
Reduce sending speed: Implement a slower sending rate, especially when warming up new IP addresses or sending to less engaged segments. Most ESPs allow you to configure sending speed.
Segment and warm up lists: For large sends, break your list into smaller segments and send to them gradually. Prioritize your most engaged subscribers first.
Clean your mailing lists: Regularly remove inactive or invalid email addresses. High bounce rates (especially hard bounces) severely impact your sender reputation and can trigger rate limits. This also helps you avoid spam traps.
Monitor sender reputation: Use tools like Google Postmaster Tools to keep an eye on your domain and IP reputation. A decline here often precedes rate limiting or blocklisting.
If the issue persists, review your email content for anything that might flag it as spam. This includes excessive links, spammy keywords, or poor formatting. Ensure your email authentication (SPF, DKIM, DMARC) records are correctly configured, as this significantly impacts how ISPs perceive your legitimacy. Misconfigurations can lead to emails being rejected outright or heavily throttled. You can explore a simple guide to DMARC, SPF, and DKIM for more information.
Finally, communicate with your email service provider. They can often provide insights into the specific reasons for rate limiting and suggest solutions, as they have a broader view of email traffic and relationships with various ISPs. They might even be able to temporarily increase your sending limits or offer dedicated IP addresses if your volume justifies it.
Preventative measures and best practices
Preventing 'rate limit exceeded' errors is far more effective than reacting to them. Proactive measures focus on building and maintaining a strong sender reputation and adhering to best practices for email sending. This involves consistent list hygiene, sending relevant content, and monitoring your email deliverability metrics regularly.
Regularly cleaning your email list is paramount. Remove hard bounces immediately and suppress soft bounces after a few attempts. Sending to unengaged subscribers or invalid addresses will quickly degrade your sender reputation, making you more susceptible to rate limits. Consider implementing a re-engagement strategy for inactive subscribers or removing them from your active lists. You can find more information on boosting email deliverability rates.
Utilize feedback loops (FBLs) from major ISPs. FBLs notify you when subscribers mark your emails as spam, allowing you to remove those recipients from your list promptly. This prevents further spam complaints and helps maintain a positive sending reputation. Consistent monitoring of your DMARC reports can also provide valuable insights into deliverability issues.
Key takeaways
In summary, a 'rate limit exceeded' message is a temporary roadblock in email delivery, indicating that you've sent too many emails too quickly for a given receiving server or your ESP's policy. While often temporary, it's a critical signal that demands immediate attention. Understanding the root cause, whether it's related to volume, reputation, or list quality, is the first step towards resolution.
By diligently checking your logs for specific error codes, adjusting your sending patterns, and maintaining a healthy, engaged subscriber list, you can mitigate these issues. Proactive measures, such as consistent list hygiene, monitoring sender reputation, and ensuring proper email authentication, are key to preventing future rate limiting. Addressing these issues promptly not only resolves current delivery problems but also strengthens your overall email program and ensures your messages reach their intended recipients reliably.
Views from the trenches
Best practices
Always segment your email lists based on engagement and send to your most active subscribers first.
Implement a gradual sending ramp-up for new IPs or domains to build a positive sending history.
Regularly monitor your bounce rates and unsubscribe requests to keep your lists clean and engaged.
Ensure all your email authentication records like SPF, DKIM, and DMARC are correctly set up and aligned.
Common pitfalls
Sending a sudden, large volume of emails without proper IP warming can immediately trigger rate limits.
Ignoring soft bounces and continuing to send to unengaged or invalid addresses can quickly damage reputation.
Not segmenting lists and sending high volumes to an entire list, including inactive subscribers, is a common mistake.
Failing to check detailed bounce logs means you miss crucial specific error messages from ISPs.
Expert tips
A spike in 'rate limit exceeded' errors, especially from a low reputation, indicates something is off.
Always check your ESP's bounce logs; they provide specific error messages from mailbox providers.
Pay close attention to error messages like 'unusual rate of unsolicited mail' or 'suspicious due to low reputation'.
If 'delivery time expired' appears, it means the ESP's delivery attempts timed out over 24-72 hours.
Expert view
Expert from Email Geeks says that rate limits are usually related to reputation-based throttling or an unexpected spike in traffic that appears suspicious due to previous send patterns.
2023-08-16 - Email Geeks
Expert view
Expert from Email Geeks notes that rate limits can mean sending too fast to one individual recipient or exceeding limits for your current IP or domain reputation.
Gmail or 
Microsoft (Outlook/Hotmail), set these limits to maintain service stability and prevent spammers from flooding their users' inboxes. When you hit a rate limit, the server will often respond with a 4XX temporary error code, indicating that you should try sending the message again later. However, repeated violations can lead to more severe consequences, such as longer deferrals or even permanent blocklisting (or blacklisting) of your sending IP address or domain.
GoDaddy or 
