How can I use Google Postmaster Tools to diagnose email throttling?
Michael Ko
Co-founder & CEO, Suped
Published 5 May 2025
Updated 18 Aug 2025
7 min read
Email throttling is a common challenge for senders, where mailbox providers temporarily delay or reject emails to manage incoming volume and protect recipients. It can be frustrating because your emails are not hard bouncing, but they are also not reaching the inbox promptly. This often leads to significant delays in message delivery and can impact critical communications.
Understanding why your emails are being throttled, and by whom, is the first step toward resolution. This is where a tool like Google Postmaster Tools becomes invaluable, especially if a significant portion of your audience uses Gmail.
Understanding email throttling
Email throttling is the intentional slowdown of email delivery by an internet service provider (ISP) or mailbox provider. Unlike a hard bounce, which indicates a permanent delivery failure, throttling is a temporary measure. This means that while your emails aren't being permanently rejected, their delivery is being delayed. The goal of throttling from the mailbox provider's perspective is to prevent mail servers from being overwhelmed, to manage spam, and to ensure network stability.
Throttling often occurs when your sending behavior deviates from established norms or triggers certain internal algorithms. This could include a sudden, sharp increase in sending volume, a rise in user complaints (even if overall spam rates remain low), or sending to a significant number of inactive or unknown email addresses. It's a signal from the mailbox provider that they are scrutinizing your traffic more closely.
The impact of throttling is primarily seen in delayed deliveries. Your email service provider (ESP) might show emails as pending or deferred for extended periods. In your bounce logs, you might see a high volume of 4xx (transient or temporary failure) errors, as opposed to 5xx (permanent failure) errors. These 4xx errors are a key indicator of throttling.
Key Google Postmaster Tools dashboards
Google Postmaster Tools offers several dashboards that are crucial for diagnosing email throttling. Each dashboard provides a unique perspective on your sending performance, allowing you to pinpoint potential issues causing delays.
IP reputation: This dashboard displays the health of your sending IP addresses. A drop from High to Medium or Low indicates that Google views your sending practices less favorably, often leading to throttling. You can click on a specific day in GPT to see the individual IPs Google observed sending mail from your domain on that day. Google assesses IP reputation per IP address, not in aggregate.
Domain reputation: Similar to IP reputation, a declining domain reputation suggests that your domain is perceived as less trustworthy. This can affect all emails sent from that domain, regardless of the sending IP. You can monitor your domain reputation in Google Postmaster Tools.
Spam rate: A high spam complaint rate is a significant red flag. Google might throttle your mail to protect its users from unwanted messages. This dashboard shows the percentage of your emails marked as spam by Gmail users. Learn more about interpreting spam rates.
Delivery errors: This dashboard provides insights into email delivery issues. While it reports on various errors, pay close attention to transient errors (like 4xx codes), which directly indicate throttling or temporary blockages. You won't typically see 5xx errors if it's purely throttling, as 5xx implies a permanent rejection.
Interpreting GPT data for throttling signs
Diagnosing email throttling with Google Postmaster Tools (GPT) involves more than just glancing at your reputation scores. It requires a detailed analysis of trends, comparing them with your sending patterns, and understanding what normal delivery looks like versus when throttling occurs.
Typical throttling indicators
Low delivery rates: A noticeable drop in the volume of emails delivered to Gmail users, as reported in the Postmaster Tools dashboards.
Increased deferred emails: Your ESP's logs show a higher count of emails deferred or temporarily rejected, often with 4xx SMTP codes.
Stable reputation: Sometimes, throttling can occur even if your IP and domain reputation in GPT appear High. This suggests a rate limit issue rather than a reputation one, for example, a sudden spike in volume.
Normal delivery signs
Consistent delivery rates: Steady volume and high delivery rates in GPT, with minimal fluctuations unless intended.
Low deferred emails: Your ESP's bounce logs show very few, if any, temporary failures.
Healthy reputation scores: Consistently high IP and domain reputation in GPT, indicating trust from Google.
One common scenario that indicates throttling is when one of your sending IPs experiences a slowdown while another continues to deliver normally, despite both having good reputation scores. This often points to specific volume limitations or other nuanced behavioral issues that GPT might not explicitly detail but which manifest as delivery delays.
Deeper insights into throttling detection
When diagnosing throttling, remember that Google Postmaster Tools aggregates data. If you have multiple IPs sending email, GPT will show collective data, but you can drill down to see reputation metrics per individual IP. This granular view is essential for identifying which specific IPs might be experiencing throttling.
Typical transient error message
When Google throttles your email, you’ll typically see a transient (4xx) SMTP error in your logs, rather than a permanent (5xx) one. These errors indicate a temporary issue, and the sending server should retry delivery later. This is different from a hard bounce, which means the email cannot be delivered.
Example SMTP 4xx Error for Throttlingtext
421 4.7.0 [IP] 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 try again later. For more information, go to https://support.google.com/mail/?p=UnsolicitedIPError a10si8696956qgc.26 - gsmtp
Another crucial point is that your ESP might not provide detailed information on all transient failures in their standard bounce reports. Many ESPs employ sophisticated back-off mechanisms, retrying messages after temporary rejections. If an ESP successfully retries and delivers an email after a transient throttling event, it might not appear as a bounce in your stats, only as a delay. To get a full picture, you may need to request detailed delivery logs from your ESP, specifically looking for transient failure attempts or records of extended delivery times for specific messages.
Preventative measures and best practices
While Google Postmaster Tools helps diagnose throttling, preventing it requires a proactive approach to your email program. Focus on maintaining excellent sender reputation, adhering to best practices, and closely monitoring your deliverability.
Maintain list hygiene: Regularly clean your email lists to remove inactive or invalid addresses. Sending to engaged recipients improves your reputation and reduces spam complaints. This is key to avoiding both throttling and getting on a blacklist (or blocklist).
Warm up new IPs/domains: If you're using new sending infrastructure, gradually increase your email volume over time. Sudden spikes can trigger throttling even for legitimate mail. Learn how to warm up IPs.
Ensure proper authentication: Verify your SPF, DKIM, and DMARC records are correctly configured. Authentication failures can negatively impact your sender reputation and lead to throttling or even blocklisting. Monitor your DKIM success rate and troubleshoot SPF failures.
Views from the trenches
Best practices
Actively use Google Postmaster Tools to track IP and domain reputation daily, not just when issues arise.
Segment your email lists to send targeted content, which can improve engagement and reduce spam complaints.
Implement a clear unsubscription process to minimize complaint rates, as Google monitors these closely.
Common pitfalls
Assuming that a good IP/domain reputation automatically prevents throttling, as sudden volume spikes can still trigger it.
Not asking your ESP for detailed transient bounce logs, which hide true throttling events by showing successful retries.
Ignoring small, consistent dips in reputation metrics, as these can escalate into significant throttling over time.
Expert tips
If your ESP says they see no issues, ask them for specific delivery logs, including retries and temporary failures, to confirm.
Monitor your delivery times for critical campaigns, as delays can be a silent indicator of throttling before it hits logs.
Consider how changes in your email sending frequency or content might affect Google's perception of your traffic.
Marketer view
Marketer from Email Geeks says they had two IPs for sending, and one was throttled for 18 hours while the other delivered normally. Their ESP thought it was throttling but had no data to back it up, so they expected to see more 4.x.x or 5.x.x bounces.
2023-08-07 - Email Geeks
Expert view
Expert from Email Geeks says that throttling can happen even with a good reputation. It's important to ask the ESP for detailed reasons why they suspect throttling is occurring.
2023-08-07 - Email Geeks
Sustaining optimal email delivery
Diagnosing email throttling relies heavily on understanding the signals from tools like Google Postmaster Tools and cross-referencing them with your ESP's data. By consistently monitoring your key email deliverability metrics, you can identify throttling early and implement strategies to ensure your emails reach Gmail inboxes efficiently. This proactive approach helps maintain a healthy sender reputation and ensures your messages are delivered when intended, preventing them from ending up on a blacklist or in the spam folder.
IP reputation per IP address, not in aggregate.
