Symptoms
When adding an email forwarder in cPanel or Webmail, you receive an error similar to the following.
The system could not add the forwarder: (Warning: “target@domain.tld” does not refer to a valid local email account or alias. The system will not create a forwarder, because it already sends that email to the default address.)
Description
When the target address of an email forwarder is at a domain listed in the /etc/localdomains file, the target address must exist on the server as an email account or a forwarded address.
Workaround
To address this issue, you must either create the target email account on the server or update the email routing of the target domain if the domain receives email at another server.
Additional resources
How can I create an email account?