4.5.4Temporarily invalid command arguments

A mail system recognized the command but rejected its arguments as invalid. The leading digit 4 classifies this concrete result as temporary, so delivery may be retried in a controlled way after the arguments are checked and, when necessary, corrected.

The X.5.4 pattern means that a valid mail transaction protocol command was issued with invalid arguments: the arguments were out of range or represented unrecognized features. The registry description says this detail is useful only as a permanent error, but the accepted concrete 4.5.4 variant has class 4; under the class-alignment rule, its result remains transient.

The leading digit 4 denotes a persistent transient failure: the current attempt failed, but the result is not a permanent rejection. The tension between class 4 and the registry note about using this detail as a permanent error makes the full response and command context important; the code alone does not show that retrying the same arguments unchanged will succeed.

Class

Temporary failure

Retry

Controlled retry

Suppression

Check the full context

Operational recommendation: before another attempt, check suppression state and inspect the exact command, its arguments, and transaction state; first correct any confirmed range or feature-compatibility defect. Then retry delivery through a controlled mechanism with backoff, jitter, idempotency, and a time or attempt limit. If 4.5.4 repeats in the same context, stop retrying and refer the case for diagnosis.

Operational recommendation: do not add the address to a suppression list based on temporary code 4.5.4 alone. Inspect the full response, command and arguments, bounded-attempt history, and other delivery events; suppress only after an independent permanent signal or when the applicable policy requires it.

• An argument to a valid mail transaction protocol command was outside the range accepted by the system that returned the code.
• An argument to a valid command represented a feature that the system returning the code did not recognize.

1. Inspect the raw SMTP response or delivery report and confirm that the enhanced code is exactly 4.5.4 and that the basic reply is in the 4xx class; retain the complete response text.
2. Correlate the response with the intended message, attempt time, system that returned the code, and exact command; retain the arguments, transaction state before the command, and response after it.
3. Use the retained trace and logs to determine whether an argument was out of range or represented an unrecognized feature; do not choose between these causes from the code alone.
4. Check suppression before retrying, correct the confirmed argument problem, and compare the results of bounded attempts; stop after success, a permanent result, or the configured limit is reached.

Compare 2.5.0, the general protocol status in the success class; temporary 4.5.0 for another or undefined protocol problem; 4.5.1 for a command issued out of sequence or unsupported; and 4.5.3 for too many recipients.

• 2.5.0Other or undefined protocol status
• 4.5.0Other or undefined temporary protocol status
• 4.5.1Temporarily invalid protocol command
• 4.5.3Too many recipients

The canonical T0 meaning of X.5.4, its note about permanent-error use, and the accepted code's class-4 assignment were verified against the IANA registry and RFC 2034, RFC 3463, and RFC 5248 as of July 17, 2026. Retry, diagnostic, and suppression guidance is presented separately as operational advice.

• iana-smtp-enhanced-status-codesT0 source

  Enumerated Status Codes / X.5.4

• 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

• rfc3463T0 source

  IANA registry reference for X.5.4

Last verified: 17 July 2026