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SMTP codes

SMTP 555 MAIL FROM/RCPT TO parameters not recognized

The server did not understand an ESMTP parameter on MAIL FROM or RCPT TO, such as SIZE or SMTPUTF8.

555
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 5321

What to do

How it appears in a bounce

555 MAIL FROM/RCPT TO parameters not recognized

Paste the full bounce into the decoder — it finds every code and recipient and says what to check.

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