3-digit reply code Permanent

SMTP error 504

Command parameter not implemented.

Updated Jul 1, 2026

The short answer

Command parameter not implemented. Common cause: A parameter you passed is not supported by the server.

Quick fix

Check the command syntax your client is using.

5 - Permanent. The server will not try again. Fix the cause before you resend.

How to fix it

Check the command syntax your client is using.

Related codes

511 Bad destination mailbox address — a misread of 5.1.1, not a real code. 513 Non-standard label: bad address syntax, or a relay/auth failure. 517 Non-standard label: bad sender attribute, or a policy block. 522 Mailbox full — a non-standard label for 552 / 5.2.2. 533 Non-standard label, usually a misread of 5.3.3. 510 Invalid recipient address — a vendor label, not a real code.
All SMTP error codes
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