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.
What Does Error 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.
Common Causes
- 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 Error 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
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).
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.
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.