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SMTP codes

SMTP 5.5.0 Protocol error

An SMTP protocol error, often a mangled command.

5.5.0
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 3463

What to do

How it appears in a bounce

550 5.5.0 Protocol error

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

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