Basic SMTP reply code · RFC 5321
555 MAIL/RCPT parameters not recognized
The MAIL FROM or RCPT TO parameters are not recognised or not implemented.
- Class: 5yz · Permanent failure
5yz: a permanent failure. The same request should not be repeated; something must change first, such as the address (RFC 5321 §4.2.1). - Source
How 555 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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555 5.5.2
Syntax error.
The SMTP command sent could not be understood (a syntax error).
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555 5.5.2
Syntax error, goodbye.
The SMTP command sent could not be understood (a syntax error).
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026