Enhanced SMTP status code · RFC 3463 · IANA
5.5.2 Syntax error
An SMTP command could not be understood: the syntax is wrong or the command is not recognised.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code is only useful as a permanent failure.
- Subject: X.5 · Protocol
Failures of the delivery protocol: implementation errors or an unreliable connection.
How 5.5.2 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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501 5.5.2
Syntax error, cannot decode response.
The SMTP command sent could not be understood (a syntax error).
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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