What does 551 mean?
SMTP 551 indicates the user is not local:
What it means:
The server you connected to does not handle mail for this recipient. The address may have moved or may have been incorrectly addressed.
Common causes:
Address changed: User moved to different server. Domain routing: Mail should go elsewhere. Configuration issue: Server does not accept for this domain.
Typical message examples:
"551 User not local" "551 Please try [forwarding address]" "551 Not our customer"
Response:
Check if forwarding address was provided. Verify you have the correct address. Suppress if address is confirmed invalid.
551 says "wrong harbor." This ship needs to dock elsewhere.
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