What causes an unusually high percentage of mailbox full email bounces?
Matthew Whittaker
Co-founder & CTO, Suped
Published 1 Jun 2025
Updated 18 Aug 2025
7 min read
When managing email campaigns, encountering bounces is a normal part of the process. However, finding that an unusually high percentage, perhaps as much as two-thirds of all bounces, are due to recipients' mailboxes being full can be a puzzling and concerning signal.
A 'mailbox full' bounce is typically classified as a soft bounce, meaning it's a temporary delivery issue. Unlike hard bounces which indicate a permanent problem like an invalid address, a soft bounce suggests the email might eventually be delivered if retried. Yet, a disproportionate number of these temporary failures points to underlying issues that need attention.
Understanding why this particular type of bounce is so prevalent in your email operations is crucial for maintaining good sender reputation and ensuring your messages reach their intended audience. It's a key indicator of list hygiene and engagement health.
What are 'mailbox full' bounces?
A 'mailbox full' bounce occurs when the recipient's email server rejects an incoming message because the allocated storage quota for that particular inbox has been exceeded. This is often a temporary state, and many email service providers (ESPs) will attempt to resend the email several times over a period, hoping the mailbox clears space.
It's important to differentiate between soft bounces and hard bounces. Soft bounces, like 'mailbox full' or 'server temporarily unavailable,' suggest a transient issue. Hard bounces, conversely, indicate a permanent problem, such as an invalid or non-existent email address, and generally require immediate removal from your mailing list.
While a few 'mailbox full' bounces are normal, an unusually high percentage is not. It signals that a significant portion of your list may be inactive, unmanaged, or experiencing persistent issues. This can negatively impact your overall email deliverability. You can learn more about general email bounces and their causes from resources like Postmark's guide on email bounces.
Soft bounces
These are temporary issues. Examples include a recipient's mailbox being full, the receiving server being temporarily down, or the message size exceeding server limits. Senders typically retry these emails.
Hard bounces
These are permanent delivery failures. Common causes include invalid email addresses, non-existent domains, or blocked email addresses. Hard bounces should result in immediate removal from your mailing list to protect your sender reputation.
Primary reasons for an unusually high percentage
One of the most common reasons for a surge in 'mailbox full' bounces is an outdated or unengaged email list. If subscribers haven't interacted with your emails in a long time, their mailboxes might accumulate unread messages, eventually reaching their storage limits. This is particularly true for older accounts or those used infrequently.
Poor list acquisition methods can also be a significant contributor. If you're acquiring email addresses through unreliable sources, or if there's an incentive that encourages people to provide throwaway or rarely checked email addresses, such as gaming a referral scheme, you'll inevitably end up with a higher number of full mailboxes. These accounts may never be logged into or cleared, leading to persistent bounce issues.
A lack of systematic bounce handling on your part can also exacerbate the problem. If your system doesn't automatically suppress or remove addresses that repeatedly generate 'mailbox full' bounces, you'll continue to send emails to them, further inflating your bounce rates. This demonstrates poor list management practices to Internet Service Providers (ISPs), impacting your reputation.
Occasionally, the issue isn't with the recipient's mailbox directly, but with the receiving mail server itself. Temporary outages, server overloads, or misconfigurations on the recipient's end can sometimes trigger 'mailbox full' errors, even if the inbox technically isn't at capacity. Additionally, very large email sizes or attachments can sometimes trigger bounces if they exceed server-imposed limits.
Broader implications for deliverability
ISPs and mailbox providers closely monitor bounce rates. A consistently high percentage of 'mailbox full' (soft) bounces, even if temporary, is a red flag. It indicates that you might be sending to an unengaged or poorly maintained list, which can lead to a degraded sender reputation. This, in turn, can cause your emails to land in the spam folder or even be blocklisted (or blacklisted) entirely.
ISPs often interpret high soft bounce rates as a sign of poor sending practices, even if you're not intentionally sending spam. They prioritize delivery for senders who maintain clean lists and have good engagement. If your soft bounces are high, your messages might be subjected to stricter filtering, leading to lower inbox placement rates across all campaigns. This affects your overall email deliverability.
It's also worth noting that major providers like Gmail have been known to adjust their enforcement policies over time, which can impact bounce rates. Even if storage limits haven't changed, how actively they manage and bounce emails to full mailboxes might have, leading to unexpected spikes in these specific bounce types. A discussion on Gmail storage limits on Reddit provides some insight into this.
Poor list hygiene
List decay: Over time, email addresses become inactive or abandoned, leading to full mailboxes.
Bad acquisition: Gathering addresses from unverified sources or through referral schemes that encourage fake sign-ups.
No suppression: Continuing to send emails to addresses that repeatedly soft bounce due to full mailboxes.
Good list hygiene
Regular cleaning: Routinely removing inactive or unengaged subscribers from your list.
Verified sign-ups: Using double opt-in processes to ensure legitimate and active addresses.
Automated handling: Implementing systems that automatically suppress addresses after a few soft bounces.
Strategies for mitigation
To reduce an unusually high percentage of 'mailbox full' bounces, the most effective strategy is to implement rigorous list hygiene practices. Regularly clean your email list by removing unengaged subscribers who haven't opened or clicked your emails in a significant period. This ensures you're sending to an active and responsive audience.
Implement robust bounce management policies within your email sending platform. Most ESPs allow you to set rules for how many times a soft bounce should be retried before the address is automatically suppressed. It's generally a good practice to stop sending to an email address after a few consecutive soft bounces, as continuing to do so can harm your sender reputation, similar to how invalid addresses impact deliverability.
Additionally, actively monitor your bounce data. Analyze patterns in your bounce reports, looking for specific domains or ISPs that are generating a disproportionate number of 'mailbox full' errors. This can help identify if the issue is widespread or concentrated, allowing you to tailor your approach. For example, a sudden spike in Gmail 'mailbox full' deferrals might suggest an issue specific to that provider's handling.
Proactive steps to reduce bounces
Implement double opt-in: Verify new subscribers immediately to ensure valid email addresses.
Regular list validation: Use a third-party service to periodically check your list for validity and deliverability.
Engagement segmentation: Segment your list and send more frequently to engaged users, less to unengaged ones.
Sunset policies: Automatically remove subscribers who haven't engaged after a certain period.
Views from the trenches
Best practices
Maintain a strict list hygiene regimen by regularly removing unengaged or inactive subscribers to minimize bounces.
Implement automated bounce handling rules within your email platform to stop sending to persistently full mailboxes.
Segment your email list based on engagement to send targeted content to active users, improving overall deliverability.
Common pitfalls
Continuing to send emails to addresses that consistently return 'mailbox full' bounces, which harms sender reputation.
Acquiring email addresses from unverified sources or through methods prone to generating low-quality sign-ups.
Failing to analyze bounce reports for patterns, such as specific domains or ISPs contributing most to 'mailbox full' errors.
Expert tips
Consider a re-engagement campaign for inactive subscribers before removing them, giving them a chance to opt back in.
Pay close attention to changes in ISP enforcement policies, as these can suddenly impact 'mailbox full' bounce rates.
Utilize email validation services to proactively clean your list and prevent sending to problematic addresses.
Marketer view
A marketer from Email Geeks observed an unusually high percentage of mailbox full bounces, accounting for roughly 66% of all received bounces in the first quarter of the year, questioning if this was typical.
April 14, 2025 - Email Geeks
Expert view
An expert from Email Geeks suggested investigating the patterns in the bouncing addresses, particularly whether the issue was concentrated at a specific Internet Service Provider (ISP), as a 1.4% bounce rate due to full mailboxes seemed unusual.
April 14, 2025 - Email Geeks
Proactive list management is key
An unusually high percentage of 'mailbox full' email bounces is a clear warning sign that your email list health and sending practices may need review. While a few such bounces are normal, consistently high numbers signal deeper issues related to subscriber engagement, list acquisition, or inadequate bounce management.
Proactive list management, including regular cleaning and robust bounce handling, is paramount. By understanding the causes and actively implementing mitigation strategies, you can significantly reduce these bounces, improve your sender reputation, and enhance your overall email deliverability. This dedication ensures your messages are not just sent, but actually received.
Gmail have been known to adjust their enforcement policies over time, which can impact bounce rates. Even if storage limits haven't changed, how actively they manage and bounce emails to full mailboxes might have, leading to unexpected spikes in these specific bounce types. A discussion on Gmail storage limits on Reddit provides some insight into this.
