Why am I seeing bounces for mailbox full followed by opens?
Michael Ko
Co-founder & CEO, Suped
Published 21 Apr 2025
Updated 16 Aug 2025
7 min read
It can be perplexing for email senders to see a bounce message indicating a recipient's mailbox is full, only to find later that the same email was opened or even clicked. This scenario often leaves many questioning the accuracy of their email metrics and the actual delivery status of their campaigns.
The immediate thought is usually that the recipient must have cleared space after the initial bounce, allowing a subsequent delivery and open to occur. However, when this happens repeatedly with the same address, or with a very short time gap, it suggests more complex underlying factors than a simple mailbox cleanup.
This situation points to a nuanced interplay of mail server behaviors, temporary delivery issues, and the often-misunderstood impact of email security systems. I'll delve into the common explanations for why you might observe mailbox full bounces followed by opens, offering clarity on this seemingly contradictory behavior.
Understanding email bounces
Understanding how email bounces are classified is crucial. A bounce is essentially a notification from a mail server indicating that an email could not be delivered. There are two primary types of bounces, soft bounces and hard bounces.
A soft bounce indicates a temporary delivery issue. Common reasons for soft bounces include the recipient's mailbox being full (a common 4.x.x error code), the mail server being temporarily unavailable, or the email size exceeding the recipient's limit. When a soft bounce occurs, your sending server, or Mail Transfer Agent (MTA), typically holds the email and attempts to retry delivery over a set period, often up to 48 hours. If the issue is resolved during this retry window (e.g., the recipient clears space), the email might successfully deliver and be opened.
However, a hard bounce signifies a permanent delivery failure, often due to an invalid or non-existent email address (a 5.x.x error code). In these cases, the sending server usually ceases further delivery attempts to that address immediately. The perplexing aspect arises when a bounce classified as a hard bounce (like some mailbox full bounces, particularly those returning a 550 error for being over quota) is followed by an open event, as this should not happen if the mail was truly rejected outright.
Common soft bounce reasons
Mailbox full: The recipient's inbox has reached its storage limit.
Server unavailable: The recipient's mail server is temporarily down or overloaded.
One of the most common reasons for seeing an open after a bounce, particularly a hard bounce, is the intervention of email security appliances. Many organizations use services like Microsoft 365 Exchange Online Protection, Mimecast, or Proofpoint. These systems are designed to scan incoming emails for malicious content before they ever reach the recipient's inbox.
During this scanning process, the security appliance may pre-fetch or pre-scan the email's content, including tracking pixels or links. This action triggers an open or click event on your tracking system, even if the email was ultimately rejected by the recipient's server due to a full mailbox.
The timing can be tricky, but it's often the security appliance initiating the 'open' right after your send, potentially before the full bounce notification is processed or even if the email is later discarded by the recipient's server due to quota limits. This results in the confusing data point of a 'mailbox full' bounce co-occurring with an 'open' event from what appears to be the same recipient.
User opens
Intentional engagement: A genuine recipient manually opens the email.
Delayed action: Occurs after successful delivery and the recipient decides to engage.
Indicates interest: A true open often suggests the content resonated with the recipient.
Scanner activity
Automated scanning: Security systems scan for malware or phishing, triggering opens.
Pre-delivery: Happens before the email reaches the actual inbox.
False positive opens: May not reflect recipient engagement.
Investigating the discrepancy
To properly diagnose the situation, the first step is always to examine the specific bounce code and message. A common mailbox full bounce message might be a 550 error indicating 'remote mailbox over quota'.
Common mailbox full bounce message
550 [internal] [oob] The message bounced due to the remote mailbox being over quota.
If you're observing an email being sent, bouncing soon after, and then opened a few hours later, especially if this pattern repeats for the same address, it strongly suggests that a security scanner is at play. The scanner initially triggers the open, but the email is then rejected by the recipient's server due to the full mailbox. Your system reports the bounce as the final delivery status, but the earlier scan has already recorded an open.
This is particularly common with corporate domains that use advanced email filtering solutions. Even if the email originates from an AT&T domain, or any other provider, if they are behind such a system, you could see this behavior. It's not that the recipient cleared space and then retrieved the email from a pending queue, but rather that the security layer processed it before the bounce was finalized.
Strategies for resolution and prevention
Addressing this issue involves a multi-pronged approach focused on improving deliverability and understanding your engagement metrics. Firstly, regularly clean your email lists to remove inactive or problematic addresses. While a mailbox full bounce might sometimes be temporary, repeated occurrences for the same address suggest it's no longer actively used or monitored, or that the quota issue is persistent.
Secondly, for addresses consistently returning mailbox full bounces, it's often best to suppress them from future sends to avoid negatively impacting your sender reputation. Even if a scanner opens the email, if the email never reaches a human's inbox, it's not truly engaging.
Lastly, understand that open rates, especially for corporate or security-conscious domains, may be inflated by automated scans. Focus more on click-through rates and actual conversions as truer indicators of engagement. While perplexing, such bounce-then-open scenarios highlight the evolving landscape of email delivery and the need for robust deliverability practices.
May still lead to opens if delivered during retry window.
Hard bounce (permanent)
Immediately remove or suppress the address from your list.
Open events after a hard bounce are typically scanner-induced.
Bounce followed by open
Investigate bounce code. Prioritize clicks over opens for true engagement.
Open metrics may be misleading due to security scanners.
Views from the trenches
Best practices
Consistently monitor your bounce logs for detailed error codes.
Segment your email lists based on engagement levels to identify inactive subscribers.
Prioritize click rates as a more reliable indicator of true engagement than opens.
Common pitfalls
Misinterpreting scanner-induced opens as genuine recipient engagement.
Failing to suppress hard-bounced addresses, harming sender reputation.
Assuming all 'mailbox full' bounces are temporary and will eventually resolve.
Expert tips
If you see this pattern with a specific domain, reach out to their postmaster.
Consider adjusting your sending cadence for segments with high temporary bounces.
Implement a strict list hygiene policy to remove unengaged subscribers.
Marketer view
Marketer from Email Geeks says that if a mailbox is full, the recipient likely cleared out space at some point, allowing the message to be delivered later.
2022-08-31 - Email Geeks
Marketer view
Marketer from Email Geeks notes that this often happens if the person was on vacation or sick, their mailbox filled up, and they cleared space upon their return.
2022-08-31 - Email Geeks
Key takeaways for deliverability
The phenomenon of seeing a mailbox full bounce followed by an open or click highlights the complexities of modern email deliverability. It's often a result of advanced email security systems pre-scanning emails before they reach the recipient's mailbox, even if the mail server ultimately rejects the message due to quota issues.
For email senders, this means that while open rates can be indicative, they should be viewed critically, especially when accompanied by bounce reports. Focusing on deeper engagement metrics, maintaining a clean email list, and understanding the nuances of bounce codes are essential practices to ensure your messages truly reach their intended audience.
Microsoft 365 Exchange Online Protection, Mimecast, or Proofpoint. These systems are designed to scan incoming emails for malicious content before they ever reach the recipient's inbox.
