MAIL FROM/RCPT TO parameters not recognized or not implemented
The 555 code is a permanent failure indicating that the server does not recognize or implement the parameters supplied with the `MAIL FROM` or `RCPT TO` commands. This is similar to 504 but specific to the mail transaction commands.
- 1The client is attempting to use an ESMTP extension on the `MAIL FROM` or `RCPT TO` command that the server does not support (e.g., `SMTPUTF8`).
- 2The parameters are syntactically valid but are not implemented by the receiving MTA.
A client that supports SMTPUTF8 tries to use it with a server that does not.
MAIL FROM:<sender@example.com> SMTPUTF8 555 5.5.4 Unsupported option: SMTPUTF8
expected output
555 MAIL FROM/RCPT TO parameters not recognized
Fix
Query Server Extensions
WHEN You control the SMTP client
// Check the EHLO response to see if an extension is supported
const features = getFeaturesFromEhlo(response);
if (features.includes("SMTPUTF8")) {
// It's safe to use the SMTPUTF8 parameter
}Why this works
Proactively checking for server support prevents the client from using unsupported parameters and getting rejected.
✕ Retry the command with the same parameters
The server has permanently rejected this parameter; it must be omitted or changed for the command to succeed.
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