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Basic SMTP reply code · RFC 5321

504 Parameter not implemented

The command exists, but one of its parameters is not implemented.

Class: 5yz · Permanent failure
5yz: a permanent failure. The same request should not be repeated; something must change first, such as the address (RFC 5321 §4.2.1).
Associated enhanced codes
The enhanced codes that usually come with this code, per the registry and the providers:

5.5.45.7.05.7.45.7.40
Source
RFC 5321

How 504 appears at providers

The exact text from the provider's documentation, as you see it in the bounce, and what it means.

Gmail

504 5.7.40Unrecognized authentication type.

The authentication mechanism used is not recognised.
504 5.7.40XOAUTH is no longer supported.

The XOAUTH mechanism is no longer supported; use OAuth 2.0.

Have the whole error message?

Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026