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.