Summary
The "mailbox full" bounce message is indeed still valid in email deliverability, though its prevalence and underlying causes have evolved. While major providers like Gmail and Outlook offer substantial storage, actual mailbox capacity can be influenced by shared storage for photos and cloud documents. Corporate email servers also frequently have stricter quotas, leading to legitimate mailbox full bounces. Understanding the nuances of these soft bounces (which indicate a temporary issue) versus potential hard bounces (which suggest a permanent problem) is crucial for maintaining a healthy sender reputation and optimizing your email campaigns. This message serves as a clear signal that the recipient cannot receive new mail, at least for the time being.
Key findings
- Still valid: "Mailbox full" remains a legitimate bounce message, contrary to some assumptions that large storage capacities have made it obsolete. It signifies that the recipient's inbox cannot accept new mail at the moment.
- Shared storage impact: For consumer email services (like Gmail or iCloud), a mailbox being full is often due to shared storage limits being reached by large files such as photos or cloud drive documents, rather than just emails. This can cause over-quota bounces.
- Corporate mailboxes: Corporate or business email servers are more likely to enforce stricter storage quotas, making "mailbox full" a common bounce reason in professional contexts.
- Temporary vs. permanent: While initially a soft bounce, persistent "mailbox full" errors can indicate an abandoned account, essentially behaving like a hard bounce over time. Mail servers (MTAs) differentiate between temporary and permanent failures.
- Bounce code example: Common bounce codes include 452-4.2.2 The email account that you tried to reach is over quota (Gmail) and 552 5.2.2 : user is over quota (iCloud), explicitly indicating storage issues. You can learn more about general email bounces from this Mailgun article on email bounces.
Key considerations
- Subscriber engagement: While a temporary issue, repeated "mailbox full" bounces suggest low subscriber engagement or even an abandoned account, indicating a need for list hygiene.
- Retry strategy: For soft bounces, email service providers (ESPs) typically retry sending for a few days. If the mailbox remains full, it should be treated as a permanent failure.
- Segmentation and suppression: Consider segmenting or suppressing addresses that consistently return "mailbox full" messages to protect your sender reputation. For more details, see our guide on managing 452 mailbox full bounces.
- Monitoring bounce rates: Continuously monitor your bounce rates and categorize bounce types to identify patterns and maintain optimal email deliverability.
What email marketers say
Email marketers widely confirm that "mailbox full" is a legitimate bounce message. While some initially question its relevance due to increased storage capacities, many have observed these bounces directly, particularly with consumer accounts like Gmail and iCloud. The consensus points to shared storage for photos and cloud documents as a primary culprit for over-quota issues, rather than just email volume. Marketers also stress the importance of distinguishing between temporary and persistent mailbox full errors, often advising to treat repeated bounces as an indication of an abandoned or inactive account that should be suppressed to maintain list health and sender reputation.
Key opinions
- Personal experience: Many marketers have personally experienced or observed "mailbox full" bounces, indicating it's not an obsolete issue.
- Google/iCloud relevance: Mailbox full bounces are particularly noted for Gmail and iCloud accounts, often linked to users exceeding their storage limits with media files (photos, videos) or Google Drive documents.
- Significant bounce reason: For some, "Gmail over quota errors" are a top bounce reason, even though they deliver a high volume of email to Gmail in general.
- Abandoned accounts: While some are genuine storage issues, a significant portion of "mailbox full" bounces are suspected to be from abandoned mailboxes. Understanding different bounce types is essential.
- Temporary nature: Initially, it's a soft bounce, implying the user might clear space. However, if persistent, it signals a lack of attention from the subscriber.
Key considerations
- Treat as permanent: A common recommendation is to treat "mailbox full" as a permanent failure (or hard bounce) unless there's a strong reason to believe otherwise, especially after a few retries. You can learn more about how Mailchimp handles soft versus hard bounces.
- Impact on deliverability: Regularly attempting to send to full mailboxes can negatively impact sender reputation and overall deliverability.
- List hygiene: Persistent bounces of this type highlight the need for robust list hygiene practices to remove inactive subscribers. This is particularly important for your overall email deliverability.
- Identifying real users: If a recent clicker receives a "mailbox full" bounce, it's more likely a temporary storage issue rather than an abandoned account, warranting a different follow-up approach.
Marketer view
A marketer from Email Geeks questions the continued validity of the "mailbox full" bounce message, noting that with modern email providers like Gmail, Outlook, and Comcast offering vast storage, it seems unlikely for a typical user to fill their inbox purely with emails. They suggest this might be a relic from 16 years ago when storage limits were much stricter.
Marketer view
An email marketer from SendLayer states that a full mailbox is a common cause of a soft bounce, which email servers will typically attempt to redeliver later. This highlights its temporary nature but also its ongoing presence as a deliverability challenge.
What the experts say
Experts in email deliverability acknowledge that the "mailbox full" bounce message is still very much a reality, particularly for corporate email servers with their often stricter quotas. While consumer services like Gmail offer generous storage, the shared nature of this storage (including photos and cloud drives) means users can indeed hit their limits. Experts advise that while initially a soft bounce, consistent "mailbox full" errors over several days strongly suggest an inactive or abandoned account. This requires a strategy that includes re-attempts for a short period, followed by suppression from the mailing list to maintain a healthy sender reputation.
Key opinions
- Corporate context: "Mailbox full" bounces are still very possible and are predominantly observed with corporate email servers, which often have more stringent storage policies than consumer services.
- Temporary vs. permanent states: Mail Transfer Agents (MTAs) have distinct temporary and permanent failure responses, and "mailbox full" can transition from a temporary soft bounce to indicating a permanent issue if persistent. For more information, see our page on handling 4xx mail errors.
- Consumer account limits: Even providers like Comcast have storage limits that can be exceeded, leading to legitimate mailbox full bounces.
- Over-quota detection: Tools tracking bounce metrics confirm a notable amount of 452-4.2.2 The email account that you tried to reach is over quota from Gmail and 552 5.2.2 : user is over quota from iCloud, often linked to shared storage overages (e.g., Google Drive/Photos).
Key considerations
- Retry attempts: For a "mailbox full" bounce, it's recommended to retry sending for a few days. An actively used address will likely clear its backlog within this period.
- Abandoned accounts: If the mailbox remains full after several days of retries, it strongly suggests the account is either abandoned or primarily used for storage like photos/videos, not active email communication. Our article on disabled mailbox bounces provides further context.
- Diverse email setups: Some users have unusual configurations where their Gmail account, for instance, is primarily for shared documents or videos, with email handled by a different system, contributing to potential storage overages without active email use.
- Follow-up strategy: Email deliverability expert MessageGears advises that for senders mailing 1-3 times weekly, suppression should be considered if they consistently see mailbox full bounces, as outlined in their article on following up on mailbox full bounces.
Expert view
An expert from Email Geeks confirms that a "mailbox full" bounce message is still very much possible, especially when dealing with corporate email servers that often have stricter storage limitations compared to large consumer providers.
Expert view
A deliverability expert from Spam Resource emphasizes that when a mailbox is full, the server sends a bounce message back to the sender rather than simply dropping the email, ensuring transparency about the delivery failure.
What the documentation says
Official documentation and technical standards confirm the validity and behavior of "mailbox full" bounce messages within the SMTP protocol. These are generally classified as soft bounces (transient failures) under the 4xx status codes, meaning the server expects the condition to be temporary and encourages retries. However, persistent 4xx errors can escalate, leading to treatment similar to a permanent failure (5xx status) if the issue is not resolved. The documentation clarifies that the system should inform the sender of such conditions to allow for proper management of mailing lists and to prevent unnecessary retransmission attempts.
Key findings
- SMTP standards: The Simple Mail Transfer Protocol (SMTP) defines specific reply codes for various delivery states, including temporary conditions like a full mailbox. These fall under 4xx transient negative completion replies.
- Temporary failure classification: A "mailbox full" bounce is typically a soft bounce (e.g., 452 4.2.2), indicating a temporary condition that might resolve itself.
- Recipient server responsibility: The recipient mail server is responsible for returning an appropriate bounce message when storage limits are exceeded, providing crucial feedback to the sending server.
- Implicit permanent failure: While technically temporary, sustained "mailbox full" bounces can, in practice, function as a permanent failure if the recipient never clears space or if the account is abandoned, similar to a mailbox reporting over quota and then receiving mail again.
Key considerations
- Retry policy: Senders should implement a retry policy for 4xx bounce codes, typically over a period of hours or days, before classifying the address as permanently undeliverable. This is an important part of managing email deliverability issues.
- Bounce handling automation: Automated systems should correctly interpret mailbox full messages to determine when a temporary error becomes a reason for suppression. For example, some systems may classify a 552 bounce message as a hard bounce.
- Sender reputation impact: Ignoring persistent mailbox full bounces can harm sender reputation, as it signals a lack of responsiveness to recipient server feedback. The SMTP RFC 5321 details these protocols.
Technical article
RFC 5321 (SMTP) documentation states that the 4xx series of SMTP reply codes indicates a transient negative completion reply, meaning the command could not be executed for a temporary reason. A common instance of this is a full mailbox, where the server is temporarily unable to accept new mail but expects the condition to resolve.
Technical article
IANA (Internet Assigned Numbers Authority) Mail Parameters documentation lists enhanced status codes, including 4.2.2 Mailbox full, explicitly identifying this as a persistent transient failure. This classification indicates that while the problem is temporary, it is not expected to be resolved without action by the recipient.
Related resources
10 resources
Related pages
How do over-quota mailboxes and soft bounces affect email deliverability and sender reputation, and what are the best practices for managing them?
Why does a Gmail address report being over quota and then start receiving mail again?
How should email marketers handle 452 mailbox full bounce codes, especially from Gmail, during email warmup?
Is it bad to email addresses that keep soft bouncing?
How should 4xx mail errors be handled and when should they be considered bounces?
What is Yahoo's policy on inactive email accounts and what bounce type is returned?
What do mailbox disabled bounces indicate about email deliverability and spam traps?
Can a hard bounced email address become deliverable again, and under what circumstances?
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