Why is there a sudden spike in Gmail 'mailbox full' bounces since August 2024?
Michael Ko
Co-founder & CEO, Suped
Published 22 May 2025
Updated 17 Aug 2025
7 min read
Over the last couple of weeks, many senders have reported an alarming surge in Gmail 'mailbox full' bounces. This trend, observed distinctly since August 2024, has impacted a wide range of email programs, irrespective of their sending platform or dedicated IP usage. It's a frustrating situation when emails that were previously delivered without issue suddenly start bouncing back with messages indicating storage problems.
The common bounce message encountered is often structured like this: 554-5.4.7 [internal] message timeout (exceeded max time, last transfail: 452-4.2.2 The recipient's inbox is out of storage space. Please direct the 452-4.2.2 recipient to 452 4.2.2 https://support.google.com/mail/?p={hash} {hash}-{base64} - gsmtp. Initially, it can be puzzling, especially when recipients confirm their mailboxes aren't actually full. This article will explain the underlying reasons for this spike and what steps you can take to manage your email deliverability effectively.
Understanding the 'Mailbox full' bounce
Typically, a 'mailbox full' bounce indicates that the recipient's email storage quota has been exceeded. These are generally classified as soft bounces, meaning they are temporary delivery failures, and email service providers (ESPs) often retry sending the message for a period. However, in this recent trend, many senders have reported these temporary deferrals being converted into hard bounces by their ESPs, leading to immediate suppression of those addresses.
The critical observation is that many of these addresses were actively engaging with emails just days before the bounces began. This suggests the issue isn't simply a matter of dormant or genuinely full inboxes. The [internal] message timeout part of the error message confirms it's a deferral that, after exceeding a maximum retry time, is then treated as a permanent failure. This reclassification by ESPs amplifies the perceived problem, converting what might have been temporary delays into seemingly permanent undeliverable addresses. You can learn more about this by checking out the Google Mail support page on fixing bounced emails.
The surge isn't isolated to a single sending platform. Reports from multiple ESPs confirm a widespread increase in Gmail mailbox full bounces, suggesting a change on Gmail's side rather than an issue originating from specific sender practices or ESP configurations. This unified observation points directly to a systemic shift from Google.
The underlying cause: Google's updated policy
The primary cause of this sudden spike is Google's updated policy on handling users who are out of quota. Previously, Gmail had a more lenient approach, often allowing messages to be accepted even if an inbox was slightly over its storage limit. However, starting around August 15th, Google began strictly enforcing these limits. This means that any recipient whose allocated storage space is full will now consistently trigger a MailboxFull error, leading to the observed increase in bounces.
It's important to remember that Google's storage quota for a Gmail account isn't solely dedicated to email. It encompasses all Google services, including Gmail, Google Drive, and Google Photos. Free accounts typically have a 15GB limit. This means a user could have a relatively empty email inbox but still exceed their quota due to a large number of photos or documents stored in their Google Drive. The new enforcement policy applies across this cumulative storage limit.
This policy update likely stems from Google's ongoing efforts to manage storage resources and potentially encourage users to upgrade to paid plans. It's a significant shift that email marketers must understand and adapt to, as it directly impacts email deliverability to a large segment of their audience. More discussions about similar issues and how they relate can be found on Gmail help forums.
Navigating the new normal for senders
This stricter enforcement presents a unique challenge for email deliverability. Since the issue is tied to individual user storage across multiple Google services, it's not always predictable which recipient will bounce. Some users may clear space and receive emails again, while others may remain over quota, leading to inconsistent bounce patterns. This makes traditional bounce management more complex, as a user who bounces on one send might receive the next.
A high bounce rate, even from 'mailbox full' errors (soft bounces), can indirectly impact your overall sender reputation. While these specific bounces may not immediately land you on a blacklist (or blocklist), a persistent pattern of undeliverable emails signals poor list hygiene to inbox providers like Gmail. This can result in your legitimate emails being routed to the spam folder or throttled. Keeping an eye on Google Postmaster Tools can provide insights into your domain's health and potential deliverability issues. You can also read more about why Gmail bounce rates are suddenly increasing.
The challenge is also that ESPs classify these 'mailbox full' (or blocklist) responses differently. Some may treat them as permanent hard bounces after a few retries, automatically suppressing the address, while others might classify them as temporary deferrals. This discrepancy across platforms adds another layer of complexity for senders trying to maintain a clean list.
Understanding the bounce type
The critical bounce message: 554-5.4.7 [internal] message timeout (exceeded max time, last transfail: 452-4.2.2 The recipient's inbox is out of storage space.... This indicates a deferred message that eventually timed out and was then categorized as a hard bounce by the ESP. This is different from a typical permanent 'user unknown' hard bounce.
Actionable steps to mitigate bounces
To effectively manage this situation, a proactive approach to list hygiene and bounce management is crucial. The goal is to identify and address the addresses that are consistently bouncing due to mailbox full errors without over-suppressing legitimate, active subscribers.
Misclassification by ESPs: Some ESPs might categorize these deferrals as permanent hard bounces, leading to premature suppression of otherwise valid addresses.
Impact on sender metrics: An inflated bounce rate can negatively affect your sender reputation and lead to increased spam filtering.
Solution: adaptive suppression strategies
Segment and monitor: Create segments of customers experiencing MailboxFull bounces. Monitor their activity and engagement over time to identify truly inactive accounts.
Temporary suppression: Implement rules for temporary suppression of addresses that bounce due to quota issues. Retry after a few days, and if they continue to bounce, then consider permanent suppression. You can learn more about Gmail 'mailbox full' deferrals here.
Consult your ESP: Work with your email service provider to understand their bounce classification policies and available suppression tools. They may offer features to create specific suppression lists based on bounce type and frequency.
The key is to fine-tune your suppression rules to differentiate between a truly defunct address and one temporarily out of quota. Continuously cleaning your list of inactive or unengaged subscribers will naturally reduce overall bounce rates. Remember that a clean email list is fundamental for maintaining a strong sender reputation and ensuring long-term email deliverability.
Conclusion
This sudden spike in Gmail 'mailbox full' bounces since August 2024 is a direct consequence of Google's stricter enforcement of user storage quotas. It's a new reality for email senders, emphasizing the need for robust deliverability practices. While it may seem like a game of 'whack-a-mole' at times due to the dynamic nature of these bounces, understanding the root cause is the first step towards effective management.
By implementing adaptive suppression strategies, closely monitoring your bounce data, and maintaining vigilant list hygiene, you can mitigate the impact of these changes on your email program. The new normal demands continuous adaptation and a deep understanding of how inbox providers are evolving their policies. Stay informed and proactive to ensure your messages reach their intended recipients.
Views from the trenches
Best practices
Implement a multi-stage suppression process for 'mailbox full' bounces, starting with temporary suppression and retries.
Regularly clean your email lists by removing consistently unengaged or bouncing addresses, regardless of bounce type.
Leverage Google Postmaster Tools to monitor your sending reputation and detect any abnormal trends in bounce rates or spam complaints.
Educate your team that Google's storage quota covers all services, not just email, for a comprehensive understanding.
Common pitfalls
Immediately suppressing addresses based on a single 'mailbox full' bounce, potentially losing active subscribers.
Failing to analyze bounce messages, leading to misclassification of temporary deferrals as permanent failures.
Ignoring the broader impact of increasing soft bounces on overall sender reputation and future deliverability.
Assuming that engaged contacts will never bounce, neglecting to verify the health of your most active segments.
Expert tips
Use email engagement metrics (opens, clicks) alongside bounce data to inform your suppression decisions.
Segment your audience based on engagement levels and sending frequency to tailor your campaigns accordingly.
Consider a re-engagement campaign for contacts who frequently show 'mailbox full' bounces before permanent suppression.
Maintain open communication with your ESP about any unusual deliverability trends you observe.
Marketer view
Marketer from Email Geeks says they started seeing abnormally high 'mailbox full' bounces from Gmail on Friday, August 16th, and observed this on two different sending platforms. They confirmed that previous sends to these now-bouncing addresses were well-delivered before the 16th.
2024-08-19 - Email Geeks
Expert view
Expert from Email Geeks says that the '554-5.4.7 [internal] message timeout' indicates a deferral that the ESP translated into a hard bounce, and that the core question is why recipients ran out of space over the weekend. They advised looking closely at the audience being mailed and the history of the bouncing addresses.
Google Drive, and 
Google Photos. Free accounts typically have a 15GB limit. This means a user could have a relatively empty email inbox but still exceed their quota due to a large number of photos or documents stored in their Google Drive. The new enforcement policy applies across this cumulative storage limit.
Gmail. This can result in your legitimate emails being routed to the spam folder or throttled. Keeping an eye on Google Postmaster Tools can provide insights into your domain's health and potential deliverability issues. You can also read more about why Gmail bounce rates are suddenly increasing.
Google's stricter enforcement of user storage quotas. It's a new reality for email senders, emphasizing the need for robust deliverability practices. While it may seem like a game of 'whack-a-mole' at times due to the dynamic nature of these bounces, understanding the root cause is the first step towards effective management.
