SMTP Error 504 - Command parameter not implemented. | Warmup Inbox
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
511Bad destination mailbox address — a misread of 5.1.1, not a real code.
513Non-standard label: bad address syntax, or a relay/auth failure.
517Non-standard label: bad sender attribute, or a policy block.
522Mailbox full — a non-standard label for 552 / 5.2.2.
533Non-standard label, usually a misread of 5.3.3.
510Invalid recipient address — a vendor label, not a real code.
Ready when you are
Most rejections come back to sender reputation.
Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.