What this code means
The server permanently rejected an authentication attempt because the client's response was longer than the buffer available for the selected SASL mechanism. Do not retry the same unchanged attempt.
Technical meaning
The X.5.6 pattern means that the AUTH command failed because the [BASE64] response sent by the client exceeded the maximum buffer size available for the currently selected SASL mechanism. The registry description says this detail is useful for both permanent and persistent transient errors; this record concerns the concrete 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 and verification
The canonical T0 meaning of X.5.6, its reference to the AUTH exchange and SASL mechanism, and the assignment of code 5.5.6 to class 5 were verified against the IANA registry and RFC 2034, RFC 4954, and RFC 5248 as of July 17, 2026. Retry, diagnostic, and suppression guidance is presented separately as operational advice; this record contains no provider-specific practice.
- iana-smtp-enhanced-status-codesT0 source
Enumerated Status Codes / X.5.6
- rfc5248T0 source
Section 2.1: registry fields and non-exclusive Associated Basic Status Code
- rfc2034T0 source
Section 4: enhanced status class agrees with SMTP reply class
- rfc4954T0 source
IANA registry reference for X.5.6
Last verified:
