Why am I seeing bounces for mailbox full followed by opens?
Michael Ko
Co-founder & CEO, Suped
Published 21 Apr 2025
Updated 12 Oct 2025
7 min read
Encountering a 'mailbox full' bounce message shortly after sending an email, only to see that same email marked as opened a few hours later, can be incredibly perplexing. It defies conventional understanding of how email delivery works. This scenario points to complexities in the email ecosystem, involving transient delivery issues, recipient mail server behavior, and the increasing role of security appliances.
The key to unraveling this mystery lies in understanding the difference between temporary and permanent delivery failures, as well as the impact of various systems that process incoming mail before it reaches the end-user.
Decoding the bounce message and email journey
When an email server returns a 'mailbox full' bounce, it typically indicates a temporary issue. These are often categorized as soft bounces, characterized by a 4.x.x SMTP error code. In such cases, your Mail Transfer Agent (MTA) will typically hold the message and retry delivery for a set period, often up to 48 hours. If the recipient clears space in their mailbox during this window, the email can then be successfully delivered.
However, the scenario becomes more puzzling when a 550 bounce code is reported, which is usually associated with a permanent failure, like mailbox not found, or mailbox over quota in this specific instance. A 550 code generally means no further delivery attempts will be made. If an open is recorded after a 550 bounce, it suggests the initial bounce report might be misleading or there's an intermediary system at play.
Understanding bounce codes
Bounce codes provide critical information about why an email was not delivered. Differentiating between temporary (soft) and permanent (hard) bounces is essential for maintaining your sender reputation. Pay close attention to SMTP error codes to diagnose these issues accurately.
One common explanation for this paradoxical behavior is the presence of email security appliances or anti-spam filters at the recipient's domain. These systems often scan emails for malicious content, attachments, and links before they ever reach the actual inbox. During this scanning process, the security appliance may trigger the open tracking pixel within your email, even if the message is ultimately rejected or quarantined due to a 'mailbox full' error.
The role of security scanners and email proxies
A security scanner (like Mimecast or similar services) might initially accept the email, scan it, and in doing so, trigger an open. If, after scanning, the system determines the recipient's mailbox is full, it then sends a bounce notification back to your sending server. This sequence explains why you might see a bounce report followed by an open, without a second delivery attempt recorded on your end. The 'open' is a false positive from a bot, not the human recipient.
Traditional bounce scenario
Email sent by sender
Recipient MTA rejects due to mailbox full
Bounce report sent to sender
No open recorded
Scenario with security scanners
Email sent by sender
Recipient security gateway accepts the message
Scanner processes email and triggers open pixel
Security gateway then attempts delivery to full mailbox
Bounce report sent to sender
This highlights a critical aspect of email deliverability: what appears as an 'open' in your metrics doesn't always correspond to human engagement. Automated systems opening emails for security checks or pre-fetching content are common, especially with modern inbox providers. This means your open rates can be inflated, making it harder to gauge actual recipient interaction.
To truly understand the situation, it's crucial to examine the full bounce message headers and cross-reference them with your email sending platform's logs. The specific SMTP error codes and diagnostic information will often provide a clearer picture. For example, if you see an internal SparkPost bounce for 'mailbox full', it points to an issue with that specific provider's handling or reporting.
Investigating the discrepancy
To get to the bottom of this, start by looking at the raw bounce logs. These logs often contain detailed information, including the exact error code and any additional messages from the receiving server. If the bounce specifies a 550 over quota message, yet an open is recorded shortly after, it's highly indicative of automated scanning. In such cases, the email was never truly delivered to the human recipient's inbox.
Example of a 550 Mailbox Full Bounce Message
550 [internal] [oob] The message bounced due to the remote mailbox being over quota.
Monitoring your DMARC reports can also provide insights into email authentication and delivery. Tools like Suped's DMARC monitoring platform aggregate these reports, offering a comprehensive view of how your emails are being handled by various mailbox providers. While DMARC primarily focuses on authentication, a sudden spike in 'mailbox full' bounces from specific domains could indicate broader issues that DMARC reports might indirectly illuminate.
For specific issues, especially with major providers, consult their postmaster pages or support channels. Microsoft Learn often has detailed information on common bounce scenarios and troubleshooting steps for issues within their ecosystem. It is also beneficial to keep your email lists clean, removing bounced addresses promptly to prevent repeated issues that can hurt your sender reputation.
Mitigating bounces and improving deliverability
Even with security scanners, reducing 'mailbox full' bounces remains important for overall email deliverability. A high rate of such bounces, even soft ones, can negatively impact your sender reputation over time. Regularly cleaning your email lists to remove invalid or persistently full mailboxes is a best practice. This also applies to understanding what causes full mailbox bounces and their recovery rate.
List hygiene: Regularly audit and clean your email lists to remove inactive or bouncing addresses.
Engagement monitoring: Focus on actual clicks and conversions rather than just opens, given the prevalence of security scanners.
Segmentation: Segment your audience and send more frequently to highly engaged users, reducing sends to less active ones.
For ongoing monitoring of email delivery and authentication, Suped provides comprehensive DMARC reporting that helps you understand how mailbox providers are handling your mail. By analyzing your DMARC reports, you can identify authentication failures, detect potential spoofing, and ensure your legitimate emails are reaching their intended destinations. Understanding how Gmail mailbox full bounces affect deliverability is crucial for maintaining a healthy sender reputation.
Views from the trenches
Best practices
Always investigate the full bounce logs to differentiate between temporary and permanent errors.
Recognize that security scanners can trigger open pixels, leading to false positives in engagement metrics.
Regularly clean your email lists to remove persistently bouncing addresses and maintain good sender reputation.
Utilize DMARC reporting tools to monitor authentication and delivery, providing deeper insights into email flow.
Common pitfalls
Misinterpreting a 'mailbox full' bounce as a permanent issue without checking the SMTP error code.
Assuming every 'open' recorded means human engagement, especially when coupled with bounce notifications.
Ignoring bounce rates, which can negatively impact sender reputation and lead to future delivery issues.
Failing to understand how recipient security appliances interact with your email before delivery.
Expert tips
For persistent issues with specific domains, check their postmaster site or contact their support team for clarification.
Focus on click-through rates and conversions as more reliable indicators of engagement than open rates alone.
Implement a robust list cleaning strategy that automatically removes addresses after a certain number of bounces.
Consider the timing of bounces and opens; a quick bounce followed by a later open often points to automated scanning.
Marketer view
Marketer from Email Geeks says: I typically observe that a mailbox full bounce means the recipient eventually cleared space and the email was delivered, then opened.
2024-03-15 - Email Geeks
Marketer view
Marketer from Email Geeks says: The sequence of a bounce soon after sending, followed by an open a few hours later, and repeated with the same address, is highly unusual if there isn't a second delivery attempt. It indicates a deeper investigation is needed into the mail server's behavior and the bounce code.
2024-03-16 - Email Geeks
Resolving confusing email delivery reports
The simultaneous occurrence of 'mailbox full' bounces and opens is a clear indicator of the intricate and often opaque nature of modern email delivery. It forces us to look beyond simple metrics and consider the full journey of an email message, from sender to recipient, including all intermediary systems. The primary culprit in these confusing scenarios is typically an automated security appliance that scans emails before they reach the final inbox, falsely triggering opens while the email itself is ultimately bounced due to a full mailbox.
To navigate these complexities, a diligent approach to monitoring bounce codes, understanding how security tools operate, and leveraging advanced reporting platforms is essential. By adopting best practices for list hygiene and utilizing tools like Suped for DMARC monitoring, you can gain clearer insights into your email performance and maintain a healthy sender reputation, ensuring your messages genuinely reach their intended audience.
