What do SMTP error codes (550, 421, etc.) actually mean for reporting?
These codes defined in RFC 5321 are the server’s direct communication. SMTP codes tell you the reason for the failure.
550 means the address does not exist or was rejected. This is a hard bounce.
421 means the server is temporarily unavailable or rate limiting. This is a soft bounce.
Codes in the five hundred series indicate permanent failure. Codes in the four hundred series indicate temporary failure.
These codes determine whether your ESP reports the event as a hard bounce, soft bounce, or deferral.
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