SMTP Response 251

Description: User not local; will forward to <REMOTE PATH>

What does this mean?

This is a good response. It means the mail server accepted your message, however the email account specified does not actually reside on that server. Thus, it is forwarding it to the correct server. An example of this would be using cPanel’s mail forwarding or having a remote MX record for a domain.

There is not a “fix” for this SMTP Response code, as it’s not specifically an error.