Why am I seeing bounces for mailbox full followed by opens?

Key findings

• Delayed bounce reports: The bounce notification might be processed and reported after the email has, in fact, been successfully delivered and opened. This delay can create a misleading timeline in your analytics.
• MTA retries: A 'mailbox full' error is usually classified as a soft bounce (a temporary issue, often indicated by a 4xx SMTP code). Mail Transfer Agents (MTAs) are designed to retry sending soft-bounced emails for a period of time (e.g., 24-72 hours). If the recipient clears space during this retry window, the email can then be delivered. You can learn more about this in our article on why you get soft bounces.
• Security scanning: Many corporate email systems and security solutions employ advanced scanning appliances that pre-fetch and open emails to check for malicious content before delivering them to the user's inbox. This can trigger an 'open' event even if the actual delivery to the user's mailbox fails or is delayed. Some of these tools might even trigger clicks. This is also a common reason why an email shows an open when the mailbox is unknown.
• Misclassified bounces: While 'mailbox full' is typically a soft bounce, some receiving mail servers might issue a hard bounce (5xx SMTP code) if their internal policy deems the quota issue more permanent or critical. This is a crucial distinction, as a hard bounce implies no further delivery attempts.

Key considerations

• Review bounce codes: Always inspect the specific SMTP bounce code and message. A 4.x.x code indicates a temporary issue, while a 5.x.x suggests a permanent failure. This is essential for proper bounce management.
• Analyze sending platform logs: Check your email service provider's (ESP) detailed logs to confirm if any retry attempts were made after the initial 'mailbox full' bounce and if those retries resulted in successful delivery.
• Understand proxy opens: Be aware that not all 'opens' represent genuine human engagement. Security software can trigger open pixels, leading to inflated or misleading metrics, especially in corporate environments.
• Segment bounce types: Properly categorize bounces as soft or hard to inform your list hygiene strategy. Repeated soft bounces to the same address may indicate a deeper issue, while hard bounces should lead to immediate suppression.

Key opinions

• Recipient action: The most common initial thought is that the recipient actively cleared space in their mailbox, allowing the email to be delivered and subsequently opened. This is often seen as a sign of an active, albeit full, inbox.
• Delayed delivery: Some marketers believe the email was queued or deferred by the receiving server and eventually delivered, with the bounce notification arriving separately or earlier than the final delivery confirmation. The differences in soft and hard bounces are important here.
• Data discrepancies: The simultaneous occurrence of a bounce and an open leads to questions about the accuracy of reporting tools and the interpretation of email metrics.
• Transient errors: Many view 'mailbox full' as a temporary state that typically resolves itself, expecting the sending system to retry delivery.

Key considerations

• List hygiene: Despite an open, persistent 'mailbox full' bounces to the same address should prompt consideration for suppression to maintain list quality and sender reputation. This is discussed in our guide on what causes full mailbox bounces.
• Suppression criteria: Establish clear rules for when to suppress addresses based on bounce types and recurrence, even if opens are recorded. This is critical for improving your email deliverability rates.
• Engagement signals: While an open is generally positive, for bounced emails, consider it alongside other metrics to gauge true recipient interest versus automated interactions.
• Follow-up strategy: Decide on a strategy for following up with addresses that show this paradoxical behavior. Some might attempt a re-send, while others might suppress after a few occurrences.

Key opinions

• Proxy opens by security scanners: A leading explanation is that email security gateways (e.g., Mimecast, Proofpoint) or antivirus software open emails to scan them for malware or phishing attempts. This automated action triggers the open pixel, regardless of whether the email reaches the end-user's inbox. This is a common reason why emails might hard bounce then open.
• Misinterpretation of 550 codes: While 'mailbox full' implies a soft bounce (4xx), some systems may return a 550 code, which is typically a hard bounce. This specific 550 code, particularly with [internal] [oob] (out-of-band) notations, can indicate a system-specific rejection, possibly by an intermediate security layer.
• ESP logging nuances: The sender's ESP might log the initial bounce report without recording subsequent implicit retries or a later successful delivery, creating a false impression of a simultaneous bounce and open.
• Recipient server policies: Different Mailbox Providers (MBPs) handle 'mailbox full' errors and retries with varying policies, affecting when and if an email is ultimately delivered.

Key considerations

• Full header analysis: A deep dive into the full bounce message and email headers can reveal the exact reason for the bounce and the journey of the email, providing clues as to whether it was truly delivered or merely scanned.
• Distinguishing real vs. proxy engagement: For accurate metrics, it's crucial to understand how your open and click tracking is affected by security software. This is a fundamental aspect of understanding your email bounce management.
• Monitor specific domains: If these anomalies are concentrated on specific domains (like AT&T, as mentioned in the original query), it could indicate particular policies or security setups on those domains. Understanding Gmail mailbox full deferrals can be a related area.
• Consult your ESP: For internal bounce codes (like SparkPost's [internal] [oob]), your Email Service Provider (ESP) is the best resource for clarification on how these are processed and what they signify.

Key findings

• Soft vs. hard bounces: Documentation typically classifies 'mailbox full' as a soft bounce (temporary failure, SMTP 4xx series), meaning the sender's Mail Transfer Agent (MTA) should retry delivery. Hard bounces (permanent failures, SMTP 5xx series) indicate an unrecoverable error. The article Hard Bounce vs. Soft Bounce Emails offers more insight.
• SMTP reply codes: Standard SMTP reply codes (RFCs) define specific meanings for delivery failures. A 452 error code, for example, often indicates 'Requested action not taken: insufficient system storage.' However, some systems might use a 5xx code with additional context for a permanent 'over quota' rejection.
• Email tracking mechanisms: The mechanism for tracking opens (usually a 1x1 pixel image load) is distinct from actual email delivery. Automated systems, like security scanners, can trigger this pixel before the email reaches the recipient's final inbox, creating a false positive for engagement.
• Non-delivery reports (NDRs): When an email bounces, the sender typically receives an NDR, or bounce message, detailing the reason for failure. The content of this message is critical for diagnosing the problem accurately. Our guide What do verbose bounce messages tell us? provides more context.

Key considerations

• Standard vs. practice: While RFCs establish standards for email protocols, how mail servers implement these standards, especially regarding temporary failures and retries, can vary significantly. This can lead to discrepancies like a 'mailbox full' being reported as a hard bounce.
• Understanding deferrals: A 'mailbox full' bounce is often a type of deferral, where the mail server temporarily declines the message, expecting a retry. This deferral can then be followed by successful delivery if the condition is resolved.
• Open tracking limitations: Documentation implicitly acknowledges that open tracking is not a foolproof indicator of human engagement, as it's susceptible to automated pre-fetching by security systems. This impacts how you interpret your overall email deliverability rate.
• Impact on sender reputation: While soft bounces are less damaging than hard bounces, a high volume of 'mailbox full' bounces, even if some eventually open, can signal low recipient engagement and potentially impact your sender reputation over time.