Summary
The observed phenomenon of receiving 'mailbox full' bounces followed by opens can be attributed to a combination of factors. Primarily, a 'mailbox full' bounce is generally categorized as a soft bounce, indicating a temporary delivery failure. As such, sending email systems, adhering to standards outlined in RFC documents and as implemented by platforms like Microsoft Exchange Online and Sparkpost, are programmed to automatically retry delivery for a defined period. If the recipient subsequently clears space in their mailbox before the retry period expires, the message can be successfully delivered and opened. However, it's also important to consider that opens may not always reflect genuine user engagement; automated scanning appliances can sometimes trigger open notifications. In specific cases, certain bounce codes might indicate an internal issue within a sending platform like Sparkpost, warranting direct investigation with their support team.
Key findings
- Soft Bounce Nature: 'Mailbox full' is categorized as a soft bounce, signaling a temporary issue.
- Automated Retries: Email systems are designed to automatically retry delivery for soft bounces.
- RFC Compliance: RFC specifications dictate retry mechanisms for temporary failures like 'mailbox full'.
- Quota Limitations: Exceeding mailbox quotas in services like Exchange Online leads to temporary bounces.
- Space Clearance: Recipients might clear mailbox space, allowing subsequent delivery attempts to succeed.
- Potential False Opens: Scanning appliances could generate open notifications without actual user interaction.
- Internal Bounces: Some bounce codes signify internal platform-specific issues requiring vendor support.
- Numerical codes matter: 4xx vs 5xx error codes determine if the bounce is a permanent or temporary issue.
Key considerations
- Review Bounce Codes: Thoroughly analyze the specific bounce codes associated with 'mailbox full' notifications.
- Assess Retry Mechanisms: Understand the retry policies and configurations of your email sending platform.
- Account for False Opens: Factor in the possibility of inflated open rates due to scanning appliance activity.
- Investigate Internal Bounces: Reach out to your email sending platform's support team for clarification on internal bounce codes.
- Recipient Behavior: Remember recipients actions can change delivery success
What email marketers say
7 marketer opinions
The phenomenon of seeing 'mailbox full' bounces followed by opens is primarily explained by the nature of 'mailbox full' errors as soft bounces. Email systems often retry delivery for these soft bounces. If the recipient clears mailbox space between the initial bounce and a subsequent retry, the email can be successfully delivered and opened. Additionally, opens may not always represent genuine user interaction; scanning appliances can trigger false opens. Finally, internal bounces should be raised with your email sending platform support team.
Key opinions
- Soft Bounce: 'Mailbox full' is typically a soft bounce, prompting retries from the sending server.
- Retries: Email systems retry delivery after soft bounces; a successful retry explains the open after the initial bounce.
- Space Cleared: Recipients may clear mailbox space between the bounce and successful delivery.
- False Opens: Opens may be triggered by scanning appliances, not actual user interaction.
- Internal Bounce: Internal bounces from services such as Sparkpost should be raised with the ESP support team.
Key considerations
- Bounce Codes: Examine specific bounce codes to differentiate between temporary (4xx) and permanent (5xx) failures.
- Retry Logic: Understand the retry logic of your email sending platform.
- Open Tracking: Be aware that open rates might be inflated due to scanning appliances.
- Recipient Behavior: Consider the recipient's potential actions in clearing their mailbox.
- ESP Support: Contact support for internal bounces.
Marketer view
Email marketer from emaildeliveryjedi.com explains it may be that the persons system opens any email for security reasons. Therefore this maybe showing up as an open.
19 Nov 2021 - emaildeliveryjedi.com
Marketer view
Email marketer from SendPulse shares that a 'mailbox full' bounce is typically a soft bounce. They mention the sending server will attempt to deliver the message again later. Opens after a bounce could indicate the recipient cleared space in their inbox between delivery attempts.
3 Sep 2022 - SendPulse
What the experts say
2 expert opinions
The reason for seeing 'mailbox full' bounces followed by opens is that 'mailbox full' is typically a temporary issue (indicated by a 4xx error code). Email systems, like the Mail Transfer Agent (MTA), are designed to hold and retry delivery for a certain period. If the mailbox clears space during this period, a subsequent delivery attempt will succeed, leading to the email being opened after the initial bounce.
Key opinions
- Temporary Failure: 'Mailbox full' is usually a temporary failure (4xx error).
- Retries: MTAs hold and retry delivery for temporary failures.
- Subsequent Success: If space is cleared, a retry can succeed, leading to an open.
- Numerical Bounce Codes: Specific numerical codes are important, such as a 4xx error code.
Key considerations
- Error Codes: Pay attention to the specific bounce codes (4xx vs. 5xx).
- Retry Period: Understand the retry period of your MTA or email service.
- Recipient Action: Consider that recipients might clear their mailboxes.
Expert view
Expert from Email Geeks explains that a "mailbox full" bounce usually means the Mail Transfer Agent (MTA) will hold the email and retry delivery for a set period, like 48 hours.
10 Feb 2022 - Email Geeks
Expert view
Expert from Spam Resource explains that the specific numerical code accompanying 'mailbox full' is crucial. A 4xx code indicates a temporary failure and retry, while a 5xx code signifies a permanent failure. Opens after a 4xx bounce are expected as retries succeed.
2 Feb 2024 - Spam Resource
What the documentation says
3 technical articles
The 'mailbox full' bounce followed by an open occurs because 'mailbox full' is treated as a soft bounce or temporary failure (4xx error code). Email systems, as per RFC specifications and implementations like those in Microsoft Exchange Online and Sparkpost, are designed to automatically retry delivery after such soft bounces. Once the recipient's mailbox has space available, the subsequent retry is successful, resulting in the email being delivered and potentially opened.
Key findings
- Soft Bounce: 'Mailbox full' is classified as a soft bounce.
- Automatic Retries: Email systems automatically retry delivery for soft bounces.
- 4xx Error: 'Mailbox full' is often represented by a 4xx error code (temporary failure).
- Quota Exceeded: Mailbox storage quotas, when exceeded, trigger bounces until space is freed.
Key considerations
- RFC Compliance: Ensure your email system complies with RFC specifications regarding retry strategies.
- Quota Management: Be aware of mailbox storage quotas and their impact on deliverability.
- Retry Configuration: Understand and potentially configure retry settings within your email sending platform.
Technical article
Documentation from Sparkpost explains that they classify 'mailbox full' as a soft bounce. Soft bounces trigger automatic retries from their system. The bounce followed by an open indicates successful delivery during a retry attempt.
24 Jul 2021 - Sparkpost
Technical article
Documentation from Microsoft explains that Exchange Online imposes mailbox storage quotas. When a user exceeds their quota, incoming messages will bounce with a 'mailbox full' error until space is made available. Delivery after space is made is expected.
18 Jul 2024 - Microsoft Learn
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