The recipient is not hosted on this mail server. The server may suggest an alternative address.
SMTP error 551 indicates the recipient isn't on this server:
The recipient's mailbox was moved to a different mail server.
You're connecting to a server that doesn't handle this domain's mail.
MX records may be pointing to an old server.
The error message may include a forwarding address. Try sending to the suggested address.
Use our MX Checker to verify the domain's mail server configuration is correct.
Reach out through other channels to get the recipient's current email address.
551 5.1.6 User not local; please try
551 User not local, please try john@newserver.com
551 Requested action not taken: mailbox not local