SMTP Error 504: Command Parameter Not Implemented

Permanent failure Medium severity Protocol RFC 5321

What it means

SMTP Error 504 means “Command Parameter Not Implemented.” The SMTP command is supported but the specific parameter or extension used is not implemented by the receiving server. This commonly occurs with unsupported authentication mechanisms or encoding parameters.

At a glance

Code504

Bounce typeHard (permanent)

SeverityMedium

CategoryProtocol

What to doSuppress the address; do not retry

StandardRFC 5321

What it looks like in your mail logs

504 5.5.4 Unrecognized authentication type

What does 504 mean?

SMTP code 504 indicates the server supports the command but not the specific parameter or argument you used. This is most commonly seen with the AUTH command when your server requests an authentication mechanism (like CRAM-MD5 or NTLM) that the receiving server does not support.

Another common scenario is using MAIL FROM with extended parameters (like SIZE or BODY=8BITMIME) that the receiving server has not implemented. Check the EHLO response to see which extensions and parameters the server advertises.

How 504 plays out

Your server attempts delivery

The recipient server returns a permanent 504 rejection

This is a hard bounce: the message will not be accepted as sent

Suppress the address and fix the root cause before resending

Where 504 sits: soft vs hard bounce

	Soft bounce (4xx)	Hard bounce (5xx)
Nature	Temporary	Permanent
SMTP class	`4xx`	`5xx`
What to do	Let it retry	Suppress the address
Recoverable?	Often	No
504 is		✓ this code

Common causes of 504

Using an unsupported authentication mechanism with AUTH command
Specifying unsupported MAIL FROM parameters (SIZE, BODY, etc.)
Requesting unsupported encoding or transfer options

How to fix 504

Check the EHLO response for supported authentication mechanisms and extensions
Configure your MTA to use a supported AUTH mechanism (PLAIN or LOGIN are widely supported)
Remove unsupported parameters from MAIL FROM command

Frequently asked questions

What does SMTP error 504 "Command parameter not implemented" mean?

SMTP error 504 means the mail server does not recognize or support a specific parameter within an SMTP command. Unlike error 502 (which rejects the entire command), 504 indicates the command itself is valid but one of its parameters is unsupported. This commonly occurs with unrecognized authentication types or when the hostname is not set to a fully qualified domain name (FQDN).

How do I fix SMTP error 504?

To fix SMTP 504, first ensure your server's hostname is set to a fully qualified domain name (FQDN) using the "hostname" command. Verify the authentication method your client is requesting matches what the server supports. Review the SMTP command that triggered the error and check its parameters against the server's documented capabilities. If the error involves authentication, try switching from a weaker method (PLAIN) to a more secure one (LOGIN, CRAM-MD5) or vice versa.

What causes SMTP 504 authentication errors?

SMTP 504 authentication errors occur when your email client requests an authentication method the server does not support. For example, if your client sends "AUTH CRAM-MD5" but the server only supports "AUTH PLAIN" and "AUTH LOGIN," the server returns 504. Check the server's EHLO response to see which AUTH methods are advertised, then configure your client to use one of those supported methods.

Related bounce codes

Sources & further reading

RFC 5321 · The standard that defines this reply code RFC 5321, Section 4.2.2 · Defines the 504 reply for a command parameter that is not implemented RFC 4954 · SMTP Service Extension for Authentication, including AUTH mechanism negotiation

Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes