SMTP Recipient Mailbox Status Codes
X.2 status codes describe conditions at the recipient mailbox or its local configuration. Read the complete code and reply before deciding whether to retry, change content, or update recipient handling.
Mailbox status can involve a temporary list-expansion condition, a full mailbox, or a message that exceeds a mailbox-specific limit. The subject alone does not establish whether the condition is temporary or permanent.
Choose the next check
- Read the class and detail togetherUse the full code to separate the temporary list-expansion result from permanent capacity or size outcomes.
- Check mailbox capacityA full mailbox is a recipient-side capacity condition, not proof that the address syntax is invalid.
- Check the mailbox-specific limitWhen a message exceeds a mailbox limit, inspect the message and destination context before a modified attempt.
Common mistakes
- Calling every X.2 condition a bad address.
- Retrying unchanged content after a mailbox-specific length limit.
- Treating a list-expansion problem as proof that every member mailbox failed.
Codes in this category
This inventory contains only reviewed codes assigned to this subject. Open a code to see its evidence, diagnostic steps and owner-specific actions.
- 4.2.0Recipient mailbox temporarily unavailable (other/undefined mailbox status)The receiving system reports that the mailbox exists but is temporarily unable to accept the message for an unspecified reason. The code belongs to the temporary class: delivery may succeed on a later attempt.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.2.1Recipient mailbox temporarily not accepting messages (receiving-rate limit)The receiving system reports that the mailbox exists but is not currently accepting messages, most often because the recipient is receiving mail faster than the system allows. The code belongs to the temporary class: the condition is expected to clear, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.2.2Recipient mailbox temporarily over its storage quotaThe receiving system reports that the mailbox is currently over its storage limit and is not accepting messages. The code belongs to the temporary class: the recipient may free up space, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.2.4Temporary mailing-list expansion problemThe receiving system recognized the destination as a mailing list but could not expand it to recipients during this attempt. The code belongs to the temporary class, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 5.2.0Generic mailbox-status rejection, evidenced as a DMARC policy failureThe receiving system reports a permanent failure under registry pattern X.2.0, IANA's catch-all "other or undefined mailbox status" detail. No class is registry-confirmed for this pattern; the only exact production evidence in this catalog is Comcast rejecting a message that failed DMARC authentication and alignment.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.2.1Recipient mailbox exists but is disabledThe receiving system reports that the mailbox exists but is disabled and is not accepting messages. The code belongs to the permanent class for this attempt: do not retry the same unchanged send.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.2.2Recipient mailbox is fullThe recipient mailbox has exceeded its assigned quota or reached physical capacity. Code 5.2.2 is a permanent failure, so do not retry the same unchanged send.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
- 5.2.3Message exceeds the mailbox limitThe message was permanently rejected because its length exceeds the administrative limit for the recipient's specific mailbox. Do not retry the unchanged send.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
Related resources
Guide
- List Management
Mailbox-full style failures are soft, not address death.
- Deliverability
Retry soft mailbox conditions; suppress only after repeats.
Incidents
- Why is our bounce rate high?
Do not inflate hard-bounce rate with mailbox soft fails.
Wojtek Blazalek
Email deliverability expert
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