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Standard SMTP codes · permanent error

5.5.4: invalid command arguments

A valid SMTP command was sent with an argument the server does not accept, because the value was out of range or named a feature it does not know.

Standard
Provider
Permanent (5xx)
Class
Sources checked
By , email deliverability consultant
Author

At a glance

Code
5.5.4
Temporary or permanent
PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
Who has to act
The sender. The fix is on the sending side: your domain, your server or your list.
What the server says
5.5.4 Invalid command arguments

What it means

With 5.5.4 the command itself is fine but what follows it is not: an unknown parameter on MAIL FROM or RCPT TO, a value outside the allowed range, or the name of an authentication mechanism the server does not support (RFC 4954 prescribes 5.5.4 for that case, with reply 504). RFC 3463 calls it useful only as a permanent error. It often appears next to reply codes 501, 504 or 555. The usual cause is software adding an extension parameter that the server did not advertise, so the fix lies in the sending configuration.

Common causes

  • An extension parameter on MAIL FROM or RCPT TO that the server never offered.
  • An AUTH command naming a mechanism the server does not support.
  • A value out of range, such as a malformed SIZE or BODY parameter.

How to fix it

  1. 01Read the reply text for the parameter it rejected.
  2. 02Compare the parameters your software sends with the extensions in the server's EHLO reply.
  3. 03Choose an authentication method the server lists after AUTH in its EHLO reply.
  4. 04Update the sending software if it adds parameters on its own.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

  1. 501Syntax error in arguments.The command was recognised, but what came with it was malformed, often an address or a host name the server cannot accept.StandardPermanent
  2. 504Command parameter not implemented.The server implements the command, but not the particular parameter or option the client asked for with it.StandardPermanent
  3. 555Parameters not recognised.The server does not recognise, or does not implement, a parameter given with MAIL FROM or RCPT TO, so that command failed.StandardPermanent
  4. 455Parameters cannot be handled now.The server cannot handle one of the parameters given with MAIL FROM or RCPT TO at the moment, and asks the sender to try again later.StandardTemporary

5.5.4 at other providers

  1. 501 5.5.4Invalid or empty HELO name.Your client greeted Gmail with an empty or invalid name in its EHLO or HELO command, so Gmail refused it and closed the connection.GmailPermanent

All 105 standard codesEvery code in the wikiPaste your own bounce

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