What causes full mailbox bounces and what is the recovery rate?
Michael Ko
Co-founder & CEO, Suped
Published 17 Apr 2025
Updated 17 Aug 2025
9 min read
Encountering a full mailbox bounce can be perplexing. It indicates that an email couldn't be delivered because the recipient's inbox has reached its storage capacity. Unlike a hard bounce, which signifies a permanent issue such as an invalid address, a mailbox full bounce is typically a temporary delivery failure, classified as a soft bounce. This means there's a good chance the recipient can receive emails again once they clear space in their inbox.
Understanding these bounces is crucial for maintaining good email deliverability and managing your sender reputation. While frustrating, they don't necessarily mean the email address is permanently invalid or that your sender reputation is at risk. It simply signals a temporary hurdle that, with the right approach, can often be overcome. The key is knowing how to interpret these bounces and what actions to take, or avoid, in response.
Understanding mailbox full bounces
A full mailbox bounce occurs when the recipient's email server rejects an incoming message because their allocated storage limit has been exceeded. This is typically a soft bounce, meaning the issue is temporary. The email server sends back an error message, often containing a specific code like 550 overquota, to inform the sending server about the delivery failure.
One common scenario leading to a full mailbox is when an individual hasn't logged into their email account for an extended period, allowing spam and old messages to accumulate. Alternatively, some email providers, like Gmail, share storage across multiple services, such as email, photos, and cloud drives. If a user's storage is depleted by large files in Google Drive or Google Photos, their email inbox can also become full, even if they regularly clear their mail.
It's important to distinguish between a soft bounce and a hard bounce. While both prevent immediate delivery, their implications are very different. Hard bounces indicate a permanent delivery failure, like an invalid or non-existent email address, and require immediate removal from your mailing list. Soft bounces, conversely, are temporary. The recipient's server might be temporarily down, the message size could be too large, or, as in this case, the mailbox is full. For more details on these distinctions, consider learning about why email bounces happen.
Common causes for a full mailbox
Mailbox storage limits are a primary cause of these bounces. Each email provider allocates a certain amount of space to user inboxes. Once this limit is reached, new emails are rejected. While users can often clear space or upgrade their storage plans, you might encounter a high percentage of mailbox full bounces if your list contains many older, less active contacts.
Temporary server issues on the recipient's end can also cause bounces that might resemble a full mailbox. For instance, a server could be overloaded or undergoing maintenance. In such cases, the bounce message might still indicate a temporary problem, and the email could be delivered successfully once the server recovers. However, the bounce message is usually quite specific about a full mailbox.
Sometimes, anti-spam filters or security measures might temporarily block an email, leading to a soft bounce. While not directly a full mailbox issue, the outcome for your sending is similar to a temporary setback. It's crucial to ensure your sending practices do not trigger these filters unnecessarily, as consistent issues can impact your overall sender reputation and potentially lead to being placed on an email blocklist (or blacklist).
Identifying bounce types
Review bounce codes: Examine the SMTP error codes in your bounce messages. Codes starting with 4xx typically indicate temporary issues (soft bounces), while 5xx usually signify permanent failures (hard bounces).
Analyze bounce messages: Look for specific phrases like "mailbox full", "over quota", or "user account is over the limit" to confirm it's a storage issue.
Recovery rate of mailbox full bounces
The good news is that full mailbox bounces often have a decent recovery rate. While an email might bounce initially, the recipient may clear their inbox or their provider might temporarily expand their storage, allowing subsequent emails to be delivered. My research indicates that within a week, nearly 20% of contacts who initially bounced due to a full mailbox later engaged with another email, meaning their inboxes were no longer full and they were receiving mail again.
Longer-term data shows even more promising results. More than 50% of these temporarily bounced addresses become active again within a few months. This highlights that these are transient issues, not permanent ones. So, when you receive a mailbox full bounce, it’s not always a signal to immediately remove the contact from your list. It often means a temporary pause is sufficient.
The recovery rate underscores the importance of not treating full mailbox bounces the same as hard bounces. While hard bounces necessitate immediate removal to protect your sender reputation, soft bounces due to a full mailbox often resolve on their own. Email service providers (ESPs) typically have built-in retry mechanisms for soft bounces, attempting delivery multiple times over a period. This gives the recipient a chance to free up space, increasing the likelihood of eventual delivery.
Timeframe
Approximate Recovery Rate (Mailbox Full Bounces)
Within 1 week
15-25%
Within 1-3 months
40-60%
After 3 months
Over 60% (decreasing gradually)
Impact on sender reputation and deliverability
While soft bounces are less damaging than hard bounces, a consistently high volume of them can still negatively impact your email deliverability and sender reputation. Mailbox providers (ISPs) pay attention to your bounce rates, even for temporary issues. High bounce rates, regardless of type, can signal that your list is not well-maintained or that your sending practices are problematic. This can lead to your emails being filtered into spam folders, or worse, your IP address or domain being added to a blocklist (or blacklist).
ISPs use various metrics to determine your sender reputation, including bounce rates. If a significant portion of your emails consistently bounce, even temporarily, it suggests that your engagement rates might be low and that recipients aren't valuing your content. This can prompt ISPs to penalize your sending, making it harder for your legitimate emails to reach the inbox. Understanding what bounce rate percentage causes deliverability problems is key.
Maintaining a clean email list is the best defense against high bounce rates and potential blocklist (or blacklist) issues. Regularly removing inactive or non-engaging subscribers, even those causing soft bounces, can significantly improve your overall deliverability. While some email service providers categorize soft bounces as non-damaging, the cumulative effect can still be detrimental. For more information about how bounces impact email deliverability, a comprehensive guide can be helpful.
Strategies for managing and preventing bounces
To effectively manage mailbox full bounces, implementing a robust list hygiene strategy is essential. This involves regularly reviewing your subscriber list and identifying addresses that consistently soft bounce. While it's tempting to keep every email address, removing chronic bouncers, even temporary ones, improves your overall list health and signals to ISPs that you're a responsible sender. You should consider what is an acceptable email bounce rate for your campaigns.
For addresses that produce mailbox full bounces, a common practice is to temporarily suppress them from future sends. Your ESP should handle automatic retries for a period. If they continue to bounce after several attempts or over a longer duration (e.g., several weeks or months), it might be time to remove them or move them to a re-engagement segment. This targeted approach helps conserve your sending resources and prevents negative reputation signals from accumulating.
Beyond suppression, consider implementing re-engagement campaigns for segments of your list that show signs of inactivity, including temporary bounces. A well-crafted re-engagement email can prompt recipients to clean their inboxes or update their preferences, reactivating them as valuable subscribers. This proactive strategy turns potential lost contacts into engaged recipients, boosting your long-term deliverability.
Auditing your list
Segment inactive subscribers: Regularly identify and segment users who haven't opened or clicked your emails in a certain timeframe, including those with recurring soft bounces.
Implement re-engagement flows: Send targeted campaigns to inactive segments to prompt them to re-engage, offering incentives or updated preferences.
Remove persistent bouncers: After a defined period of consecutive soft bounces, consider removing the email address from your active mailing list to protect your sender reputation.
Final thoughts on managing mailbox full bounces
Mailbox full bounces are temporary and often resolve themselves quickly. While they are not as detrimental as hard bounces, consistent occurrences can still impact your sender reputation over time. By understanding their causes, monitoring their recovery, and implementing sound list management strategies, you can minimize their negative impact and maintain optimal email deliverability. Remember to differentiate between temporary and permanent bounces, adjusting your list hygiene practices accordingly to keep your email program healthy.
Views from the trenches
Best practices
Monitor soft bounce rates closely to identify trends and potential underlying issues with your email list.
Implement a retry strategy for soft bounces, allowing sufficient time for recipients to clear their inboxes.
Regularly segment inactive subscribers and consider re-engagement campaigns before outright removal.
Educate your team on the differences between hard and soft bounces to ensure appropriate list management actions.
Leverage email analytics to track recipient engagement following a soft bounce, informing your re-send decisions.
Common pitfalls
Treating mailbox full bounces as hard bounces and immediately removing addresses from your list.
Ignoring high volumes of soft bounces, which can cumulatively affect sender reputation.
Not having a defined strategy for re-engaging or suppressing contacts who consistently soft bounce.
Overlooking shared storage issues (e.g., Gmail) as a common cause for temporary full mailboxes.
Failing to communicate with subscribers about inactive accounts or low storage, missing opportunities for re-engagement.
Expert tips
Always prioritize list hygiene; a clean list is your best defense against all types of bounces.
Consider a phased approach for re-sending to soft bounces, starting with less frequent attempts.
Use engagement metrics, not just bounce rates, to assess the true health of your email list.
Keep an eye on bounce messages for any shifts in common causes, indicating new deliverability challenges.
Automate the process of suppressing or re-engaging bounced addresses to ensure consistency and efficiency.
Expert view
Expert from Email Geeks says: Mailbox full bounces are a common occurrence, and research shows a significant recovery rate for these types of temporary failures.
2023-10-26 - Email Geeks
Expert view
Expert from Email Geeks says: Gmail accounts often reach their storage limits due to shared space with services like Google Photos, leading to more frequent temporary over-quota situations that users often clean up.
Gmail, share storage across multiple services, such as email, photos, and cloud drives. If a user's storage is depleted by large files in 
Google Drive or 
Google Photos, their email inbox can also become full, even if they regularly clear their mail.
email service providers categorize soft bounces as non-damaging, the cumulative effect can still be detrimental. For more information about how bounces impact email deliverability, a comprehensive guide can be helpful.
