504
SMTPERRORCommonClient ErrorHIGH confidence
Command parameter not implemented
What this means
The 504 code means a parameter of a recognized command is not implemented. While the command itself is supported, a specific argument or option is not.
Why it happens
- 1The client is trying to use an ESMTP extension parameter that the server doesn't support (e.g., an unsupported `AUTH` mechanism).
- 2The parameter is valid according to RFCs but has been disabled on the server.
How to reproduce
A client attempts to use an authentication mechanism that the server does not support.
trigger — this will error
trigger — this will error
AUTH GSSAPI 504 5.5.4 Unrecognized authentication type
expected output
504 Command parameter not implemented
Fix
Check server features for parameter support
WHEN When you control the client software
Check server features for parameter support
// Parse EHLO response for supported AUTH mechanisms
const supportedAuth = getSupportedAuth(ehloResponse);
if (supportedAuth.includes(chosenMechanism)) {
// Proceed with AUTH
}Why this works
By checking the `EHLO` response, the client can determine which parameters (like specific `AUTH` types) are supported before attempting to use them.
What not to do
✕ Retry the same command with the same parameter
The server has permanently rejected this parameter. Retrying will not work.
Sources
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