Summary
The consensus among experts, email marketers, and technical documentation indicates that a 'mailbox full' bounce can stem from various factors. The most direct explanation is that the recipient's mailbox has genuinely reached its storage limit. However, other contributing elements include potential Gmail-specific issues (past incorrect responses, theoretical "cleanups"), server-related problems (Gmail or recipient server being unavailable), aggressive greylisting implementations misinterpreting temporary failures, incorrect email address formatting, large email sizes, and idle user accounts. Differentiating between soft and hard bounces is crucial for determining appropriate actions, such as resending or removing addresses from the mailing list to maintain sending reputation.
Key findings
- Storage Limit: The recipient's mailbox may have reached its storage quota.
- Gmail Issues: Past instances of incorrect Gmail responses or potential 'cleanups' can impact bounce rates.
- Server Availability: Both Gmail's and the recipient's server being temporarily unavailable can lead to bounces.
- Greylisting Policies: Aggressive greylisting can misinterpret temporary failures as permanent issues.
- Incorrect Formatting: Typos or incorrect formatting of email addresses contribute to 'mailbox full' bounces.
- Email Size: Large email sizes may cause delivery failures.
- Idle Accounts: Accounts that have become idle may have reached mailbox capacity.
- SMTP Codes: Mailbox full is a common reason for getting bounce backs from ISPs and uses error codes 4.2.2 or 5.2.2
Key considerations
- Bounce Type: Distinguish between soft (temporary) and hard (permanent) bounces to determine the next steps.
- List Hygiene: Regularly clean email lists by removing addresses with repeated bounces.
- Resend Attempts: Try resending emails that resulted in soft bounces.
- Analyze NDRs: Carefully analyze Non-Delivery Reports (NDRs) to understand the specific reasons for bounce failures.
- Monitor Timeframes: Monitor timeframes and circumstances surrounding bounce events to identify patterns.
- Account Activity: Review whether or not the user is actively engaging with emails.
What email marketers say
7 marketer opinions
Several factors contribute to Gmail incorrectly marking emails as bounced due to a full mailbox quota. These include the recipient's inbox genuinely reaching its storage limit, temporary server issues (either on Gmail's end or the recipient's), incorrect email address formatting, and aggressive greylisting implementations. Full mailboxes typically trigger soft bounces, suggesting a temporary issue, but repeated bounces from the same address should prompt removal from the mailing list to maintain sending reputation.
Key opinions
- Full Mailbox: The recipient's inbox has reached its storage limit.
- Server Issues: Gmail or the recipient's server may be temporarily unavailable or too busy.
- Incorrect Formatting: Typos or incorrect formatting in the email address can trigger a 'mailbox full' error.
- Greylisting: Aggressive greylisting can misinterpret temporary failures as permanent mailbox issues.
- Account Idle: Full mailboxes can often occur due to an account becoming idle. This typically occurs when users sign up for promotional offers.
Key considerations
- Soft vs. Hard Bounce: 'Mailbox full' is usually a soft bounce, suggesting a temporary issue; hard bounces indicate permanent problems.
- Resending: Attempt to resend emails that result in a soft bounce.
- List Hygiene: Regularly clean your email list by removing addresses that repeatedly bounce to avoid damaging your sending reputation.
- Email Size: Large email sizes can trigger bounces; consider reducing email size.
Marketer view
Email marketer from Hubspot explains that a full mailbox can lead to a soft bounce. They say to try resending the email. If the issue persists, clean your email list by removing those that repeatedly bounce to preserve your sending reputation.
29 Apr 2025 - HubSpot
Marketer view
Email marketer from MailerLite explains that a full mailbox can cause soft bounces. While it might be a temporary issue, repeated bounces from the same address should be removed from the mailing list.
1 Oct 2024 - MailerLite
What the experts say
5 expert opinions
Experts suggest that 'mailbox full' bounces, while seemingly straightforward, can be influenced by various factors. Some argue that the bounce accurately reflects the recipient's mailbox being at capacity, while others point to potential Gmail-related issues, like past instances of incorrect responses or theoretical "cleanups" impacting mailbox availability. Aggressive greylisting implementations can also contribute by misinterpreting temporary failures as permanent full mailbox issues. Mailbox full is considered a common reason for bounce backs from ISPs.
Key opinions
- Mailbox Capacity: The recipient's mailbox may genuinely be at its storage limit.
- Gmail Issues: Gmail may have given incorrect bounce responses in the past, potentially due to changes or "cleanups."
- Greylisting Impact: Aggressive greylisting can misinterpret temporary failures as permanent mailbox full issues.
- ISPs Bounce: Mailbox full is a common reason for bounce backs from ISPs.
Key considerations
- Verification: Investigate the timeframe and specific circumstances surrounding bounce events to determine if Gmail-specific issues are at play.
- Greylisting Policies: Be aware of how aggressive greylisting policies might affect deliverability and lead to false 'mailbox full' bounces.
- Email Opens: Consider whether email opens are genuine user actions or potentially automated processes that could affect the analysis of bounce rates.
Expert view
Expert from Email Geeks explains that there’s nothing incorrect with a ‘mailbox full’ bounce. The mailbox is full, it can’t accept mail.
4 Feb 2023 - Email Geeks
Expert view
Expert from Email Geeks explains that Gmail has given incorrect responses in the past and asks for the timeframe of the issue. He suggests the opens could have been Gmail opening the email, and asks if the users were also clicking.
27 Jul 2022 - Email Geeks
What the documentation says
4 technical articles
Technical documentation outlines that a 'mailbox full' bounce typically indicates the recipient's mailbox has reached its storage limit. SMTP protocols use specific error codes (4.2.2 or 5.2.2) to denote temporary or permanent capacity issues. Google Workspace categorizes bounces as soft (temporary, like a full mailbox) or hard (permanent), with repeated hard bounces potentially leading to removal from mailing lists. Standardized bounce message formats include diagnostic codes and details about the reason for delivery failure.
Key findings
- SMTP Codes: SMTP uses 4.2.2 or 5.2.2 error codes to indicate 'mailbox full' errors, representing temporary or permanent issues.
- Storage Limit: 'Mailbox full' NDRs mean the recipient's mailbox has exceeded its storage limit.
- Bounce Categories: Google Workspace categorizes bounces as soft (temporary, like 'mailbox full') or hard (permanent).
- Diagnostic Codes: Standardized bounce messages include diagnostic codes detailing the reason for delivery failure, including 'quota exceeded' or 'mailbox full'.
Key considerations
- Soft vs. Hard: Distinguish between soft and hard bounces to determine appropriate actions (resending vs. removal).
- List Maintenance: Implement a process for removing addresses with repeated hard bounces to maintain sending reputation.
- NDR Analysis: Analyze Non-Delivery Reports (NDRs) to understand the specific reasons for bounce failures.
Technical article
Documentation from Microsoft explains that a non-delivery report (NDR) indicating 'mailbox full' means the recipient's mailbox has exceeded its storage limit. The server is unable to accept any more emails until the recipient deletes some emails or increases the mailbox quota.
5 Sep 2022 - Microsoft
Technical article
Documentation from RFC Editor explains that SMTP defines specific codes for delivery status notifications. A 'mailbox full' error typically corresponds to a 4.2.2 or 5.2.2 error code, indicating temporary or permanent mailbox capacity issues respectively.
19 Jan 2023 - RFC Editor
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