TL;DR
Permanent for this message: one mailbox carries a length limit below the system maximum, so no unchanged retry. The sender reduces the transmitted message, or the recipient's administrator confirms a changed limit.
What this code means
X.2.3 denotes a message length limit that applies to one specific mailbox and sits below the general system maximum. It is permanent-only, and 5.2.3 is its class-5 form; the registry associates X.2.3 with basic reply 552 and records that association as non-exclusive, so other 5yz replies can carry the same enhanced code.
Provider examples
552-5.2.3 Your message exceeded Google's message size limits. Please visit http://mail.google.com/support/bin/answer.py?answer=8770 to review our size guidelines.Technical meaning
The condition compares this message with a limit configured for the destination mailbox. It is not a comparison against the receiving system's general size maximum and not a report of remaining mailbox capacity. It does not carry a numeric limit, does not identify which part of the message exceeded it, and says nothing about any other mailbox on the same domain.
Delivery status
Class 5 is permanent for the unchanged request, and RFC 2034 requires the enhanced class to agree with the class of the SMTP reply carrying it, so 5.2.3 always arrives on a 5yz reply. The permanence applies to this message at this mailbox. It does not prove the address is bad, that the mailbox rejects all mail, or that a smaller message will fail.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Stop unchanged retries. A new send is authorised only when the transmitted message is measured below the limit applied to that mailbox, or when the recipient's administrator confirms the limit has changed. Sending the same content to a different address is a new route, not a retry of this attempt.
Suppression decision
5.2.3 evaluates the size of one message against one mailbox's limit. It does not evaluate whether the address exists, whether it accepts mail, or whether it is in use. Do not suppress the recipient automatically on this code; inspect the full response and the applicable policy, and record the failure against the message rather than against the address.
Common causes
- An administrative message length limit is configured for that destination mailbox, below the receiving system's general maximum, and the transmitted message exceeds that mailbox-specific value.
- The message was sized against the wrong limit: it sits inside the receiving system's general maximum, and inside the sender's own sending limit, but above the allowance attached to this one mailbox. X.2.3 is defined for a per-mailbox limit that sits below the general system maximum, so acceptance by other mailboxes on the same system does not establish that this mailbox will accept the same message.
Diagnostic steps
- Confirm the exact enhanced code in the reply and which basic reply carried it. The registry associates X.2.3 with basic reply 552 and records that association as non-exclusive, and RFC 2034 requires the enhanced class to match the reply class, so any 5yz reply may carry 5.2.3.
- Identify which system returned the reply and retain the evidence packet: the full response text, the recipient address, the message-id, the attempt time in UTC, and the size of the message as transmitted, which is the size the receiving system compared against the limit.
- Read the full response text to separate the candidate causes, then compare behaviour across recipients: send the same message, or an equivalent test message of the same measured size, to another mailbox in the same domain. A rejection confined to one address indicates a limit attached to that mailbox; a rejection that applies to every address in the domain indicates a system-wide size limit reported under a different code.
- State the single condition that authorises a new send: ask the recipient's administrator to confirm the message length limit applied to that mailbox, and send again only when the measured transmitted size is below the confirmed value, or when the administrator confirms the limit has been raised.
Actions by owner
Sender
- Stop unchanged retries to that address and measure the message as transmitted, so the size being compared is the size the receiving system counted.
- Reduce the message for that recipient by removing the largest parts or sharing them by link, then record the new measured size and send the reduced message to that address only.
Recipient
- Confirm the intended address, obtain the attempt time and measured size from the sender, and check whether other mailboxes on the same domain received the same message.
- Ask the mailbox administrator for the message length limit applied to this mailbox, pass the confirmed value to the sender, and agree an alternative route for content that cannot fit it.
Recipient administrator
- Inspect the per-mailbox configuration and the receiving logs for that address, and confirm whether a mailbox-level length limit below the general system maximum was applied to the rejected attempt.
- State the applied limit value to the recipient, and either raise it where policy permits or confirm that it stands, so the sender can size the message against a known figure.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- SMTP Enhanced Status Codes — IANA registry of enhanced mail system status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- RFC 2034 — SMTP Service Extension for Returning Enhanced Error Codes — Defines how SMTP returns enhanced status codes to clients.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- SMTP Field Manual (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
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