SMTP
251 User Not Local
The recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient.
When You See This Code
When sending to a recipient whose mailbox is hosted on a different server. The current server knows the forwarding address and will relay the message on your behalf.
How to Fix
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.