What does 504 mean?
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You've sent a command the server understands, but the specific option you attached to it? Not so much. That's a 504 in a nutshell: the command exists, but that particular parameter isn't implemented on that server.
The most common place you'll run into this is authentication. Your mail client or sending app tries to use an AUTH method (like CRAM-MD5 or NTLM) that the receiving server doesn't support. The server knows what AUTH is. It just doesn't recognize that specific flavor.
You might see messages like "504 Command parameter not implemented", "504 Authentication mechanism not supported", or "504 Unrecognized authentication type". Different servers, same underlying story.
The fix is usually straightforward. First, check which authentication mechanisms the server actually supports. You can do this by sending an EHLO command, which lists the server's capabilities. Then switch to one it accepts, typically LOGIN or PLAIN for most modern setups.
If you're configuring a relay or SMTP connection and keep hitting 504 errors, it's worth reviewing your ESP's documentation for supported auth methods. What works on one server won't always work on another, and the 504 is just the server's polite way of saying "try something else".
Need a quick way to debug what's happening at the header level? Our free Email Header Analyzer can help you see exactly what's being negotiated during delivery.
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