5.5.4SMTP 5.5.4: Invalid command arguments

Rejected arguments on an otherwise syntactically valid mail-transaction command yield code 5.5.4 under pattern X.5.4: the arguments were out of acceptable range or named an unrecognized feature. Permanent refusal for this message is what class 5 denotes. The code does not expose the command name, offending argument, or whether range or feature mismatch triggered the reply, and it does not invalidate the recipient address.

Invalid arguments on an otherwise valid mail transaction protocol command define X.5.4: the arguments were out of range or represented unrecognized features. The T0 registry says this detail is useful only as a permanent error, and the concrete 5.5.4 code has the corresponding class 5.

The leading digit 5 denotes a permanent failure for the current message in this context. The code alone identifies neither the exact command nor argument, does not distinguish an out-of-range value from an unrecognized feature, and does not establish that the recipient address is invalid.

Class

Permanent failure

Retry

Do not retry unchanged

Suppression

Check the full context

Operational recommendation: stop automatic and manual retries of the same unchanged attempt. Consider a new send only after the argument or feature compatibility has demonstrably been corrected, or the relevant configuration has materially changed; first check suppression state again and use idempotency.

Operational recommendation: do not add the address to a suppression list based on 5.5.4 alone because the code describes a protocol-command argument, not mailbox status. Inspect the full response, command, arguments, system returning the code, and other permanent events, then suppress only according to the established cause and applicable policy.

• 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 5.5.4 and that the basic reply is in the 5xx class; retain the complete original response text.
2. Correlate the response with the intended message, attempt time, system that returned the code, and exact command and arguments; also retain the transaction state before the command.
3. Use the trace, logs, and documented system capabilities to determine whether an argument was out of range or represented an unrecognized feature. If the response identifies HELO/EHLO, inspect an empty or invalid domain argument and, in the context of the accepted Microsoft example, connection restrictions as well; do not generalize those practices to every 5.5.4 case.
4. Stop unchanged retries; before any new send, confirm that the established cause has been corrected and check suppression state again.