Summary
Full mailbox bounces, often indicated by a 550 overquota error, are a common type of soft bounce in email deliverability. They occur when a recipient's inbox has reached its storage limit, temporarily preventing new emails from being delivered. While frustrating for senders, these bounces are generally not permanent failures and often have a high recovery rate, meaning the recipient may soon clear space and become receptive to emails again. Understanding their temporary nature and managing them correctly is crucial for maintaining good sender reputation and optimizing email campaigns.
Key findings
- Temporary issue: A full mailbox bounce is a soft bounce, indicating a temporary inability to deliver, unlike hard bounces which are permanent.
- Shared storage impact: Services like Gmail share storage across multiple services (e.g., photos, drive), making mailboxes more susceptible to quickly becoming over quota.
- High recovery rate: Research indicates a significant percentage of recipients who initially bounce due to a full mailbox will engage with subsequent emails within a short period.
- Sender behavior flexibility: Unlike hard bounces, retrying delivery to addresses that produced a full mailbox bounce, potentially after a cool-down period, is generally not detrimental to sender reputation.
Key considerations
- Monitoring is key: Regularly monitor your bounce reports to distinguish between soft and hard bounces, as this informs your list management strategy.
- Strategic retries: Consider implementing a strategy to re-attempt delivery to full mailbox bounces after a reasonable interval (e.g., 24-48 hours) rather than immediately suppressing them.
- Impact on sender reputation: While single soft bounces have minimal impact, persistent soft bounces to the same address could signal an inactive recipient, which might eventually affect your sender reputation if not managed.
- List hygiene importance: Even for temporary issues, regular list cleaning and segmentation can help ensure you're sending to engaged recipients, minimizing all bounce types. Mailgun provides additional insights into how to handle email bounces.
What email marketers say
Email marketers often encounter full mailbox bounces and typically view them as temporary nuisances rather than severe threats to deliverability. Their experiences suggest that these bounces are often resolved quickly by recipients, especially with consumer-facing email services. The main challenge lies in deciding when and how often to retry sending to these addresses without negatively impacting overall campaign performance or sender health.
Key opinions
- Gmail's unique storage: Marketers frequently note that Gmail accounts are prone to over-quota bounces because storage is shared with other Google services like Photos and Drive.
- Expect recovery: There's a general consensus that many recipients will clear their inboxes or upgrade storage, making the email address active again relatively quickly.
- Temporary nature: Marketers categorize these as soft bounces, implying that repeated attempts are often warranted before full suppression.
Key considerations
- Retry strategy: Marketers debate the optimal cool-down period and number of retries before sidelining an address that consistently returns full mailbox bounces. The specific strategy can vary between brands and ESPs.
- Impact on deliverability metrics: While mailbox full bounces are soft, a persistently high rate could still signal an issue with list engagement to mailbox providers, affecting overall inbox placement.
- Subscriber experience: Some marketers suggest that repeated bounces, even soft ones, could indicate a less engaged subscriber, prompting a review of the subscriber's value. Mailchimp elaborates on soft versus hard bounces.
Marketer view
Email marketer from Email Geeks reports that their customer successfully emailed a contact from Gmail shortly after receiving a 'mailbox overquota' bounce, indicating the contact likely cleaned up their inbox.
Marketer view
Marketer from SendLayer highlights that a recipient's full mailbox is a temporary issue and not the sender's fault, emphasizing its temporary nature as a soft bounce.
What the experts say
Deliverability experts generally agree that full mailbox bounces are a benign form of soft bounce with a high likelihood of recovery. They emphasize the importance of data-driven decisions for managing these bounces, advocating for retries over immediate suppression. Experts also highlight how modern email services, particularly those with shared storage, contribute to the frequency of these temporary over-quota situations.
Key opinions
- High recovery rate confirmed: Expert research supports that a significant percentage of addresses bouncing due to a full mailbox will become active and engage with subsequent emails, sometimes over 50% within a few months.
- Not a permanent failure: Experts stress that these are not permanent failures, and retrying delivery after a short cool-down period is a sound strategy.
- Gmail's multi-service storage: Deliverability experts acknowledge that Gmail accounts are more prone to mailbox full issues due to shared storage with other Google services, a factor often overlooked.
Key considerations
- Data-driven decisions: Rely on bounce data and engagement metrics to guide when to re-attempt sending or when to consider suppressing an address.
- Contextual approach: The optimal strategy for managing mailbox full bounces can vary based on the sender's brand, the email service provider (ESP), and specific campaign goals. Assessing if a 'mailbox full' message is still valid requires careful consideration.
- Avoid premature suppression: Experts advise against immediately removing addresses that soft bounce due to a full mailbox, as this means missing out on potential future engagement. Twilio provides a deeper dive into email bounce management.
Expert view
Deliverability expert from Email Geeks notes that mailbox full bounces are somewhat common and mentions past research showing a significant recovery rate for these types of bounces.
Expert view
Expert from SpamResource emphasizes that soft bounces, like mailbox full, are not usually indicative of a permanent issue and should be treated differently from hard bounces.
What the documentation says
Email documentation, including RFCs and postmaster guidelines, categorizes mailbox full bounces as temporary, transient failures. These are typically indicated by 4xx or specific 5xx SMTP codes, like 550 overquota, which implies a temporary condition that could resolve itself. The documentation suggests that senders should generally retry sending these messages, adhering to a defined retry schedule before considering the address permanently undeliverable.
Key findings
- SMTP response codes: A full mailbox bounce is typically signaled by an SMTP reply code like 452 (mailbox full), or in some cases a 550 with a specific overquota message, which is technically a temporary failure despite the 5xx prefix.
- Retry policy: Standard email protocols recommend that sending mail servers should attempt to re-deliver messages after a temporary failure, with decreasing frequency over time.
- Distinction from permanent failures: Documentation distinguishes temporary (soft) bounces from permanent (hard) bounces by the expectation that temporary conditions can resolve on their own.
Key considerations
- Sender responsibility: While temporary, consistent full mailbox bounces over an extended period may indicate an unmaintained or abandoned account, prompting senders to consider suppressing the address. Understanding how over-quota mailboxes affect sender reputation is important.
- Postmaster guidelines: Mailbox providers often provide specific guidelines or bounce codes in their postmaster sites that can offer more granular detail on how to interpret and respond to full mailbox bounces.
- Bounce handling policy: Email service providers (ESPs) and senders should implement robust bounce handling mechanisms that differentiate between temporary and permanent failures, ensuring appropriate re-attempt or suppression actions. For more information on why email bounces happen, consult industry resources.
Technical article
The RFC 5321 (Simple Mail Transfer Protocol) states that a 452 SMTP reply code signifies a temporary transmission failure due to insufficient system storage, advising the sender to retry later.
Technical article
Google Postmaster Tools documentation indicates that while 5xx errors are generally permanent, specific codes like those for overquota can sometimes be transient and resolve over time.
Related resources
15 resources
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