TL;DR
SMTP authentication failed permanently because the client's SASL response exceeded the server's buffer for that mechanism. Shorten credentials or switch mechanisms: do not retry unchanged. Permanent; misconfigured auth loops waste sends without deliverability gain.
What this code means
AUTH exchanges fail under code 5.5.6 when pattern X.5.6 applies: the client's BASE64 response exceeds the buffer limit for the active SASL mechanism. The first digit classifies the outcome as permanent class 5. The status reports size rejection only; it does not explain why the payload grew that long, which mechanism limits apply, or whether client formatting or server buffer policy must change.
Technical meaning
AUTH failed because the client's [BASE64] response exceeded the maximum buffer size available for the currently selected SASL mechanism: that is X.5.6. The registry description says this detail is useful for both permanent and persistent transient errors; this record concerns the concrete permanent class-5 variant 5.5.6.
Delivery status
The leading digit 5 denotes a permanent failure of the current attempt. The code alone does not explain why the client response has that length or establish whether the client, configuration, or server-side handling needs correction.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Operational recommendation: stop automatic and manual retries of the unchanged AUTH attempt. Consider a new attempt only after confirming that the client response fits the buffer for the selected SASL mechanism or that the relevant server-side handling has genuinely changed; check suppression state again before the attempt.
Suppression decision
Operational recommendation: do not add the recipient address to a suppression list based on 5.5.6 alone because the code describes an AUTH exchange, not mailbox status. Inspect the full response context, authentication stage, system returning the code, event history, and applicable policy before deciding on suppression.
Common causes
- During the AUTH exchange, the client sent a [BASE64] response longer than the maximum buffer available for the currently selected SASL mechanism.
Diagnostic steps
- Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 5.5.6, the basic reply is in the 5xx class, and the failure occurred during the AUTH command.
- Correlate the response with the intended attempt, time, client, and selected SASL mechanism; retain the code, stage, and safe diagnostic metadata without storing authentication material.
- Use logs and configuration to compare the client-response length with the maximum buffer available for the selected mechanism; do not assign the precise reason for the excess length from the code alone.
- Stop unchanged retries; before a new attempt, confirm that the excess has been removed or handling has genuinely changed, then check suppression state again.
Actions by owner
Sender administrator
- Retain the full code, time, AUTH stage, selected SASL mechanism, and response length without storing its sensitive contents, then inspect how the client constructs the response.
- Stop unchanged retries and correct the confirmed cause of the oversized response; permit a new attempt only after checking it against the limit and reassessing suppression.
Provider
- For the specified time and system, inspect AUTH logs, the selected SASL mechanism, and the available buffer limit to confirm where and why the rejection occurred without exposing authentication material.
- Correct any confirmed problem in the managed configuration or implementation, or give the sender administrator safe information about the applicable constraint; preserve the exact code and response context.
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 4954 — SMTP Service Extension for Authentication — SMTP AUTH extension and related reply status codes.
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