What is the Relaying Denied Error?
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An open relay is generally understood to be a server that allows emails from any sender to any recipient without the need for authentication.
This represents a considerable risk because such servers are often used by third parties to send SPAM. For this reason, most providers (including) do not accept emails from servers known as open relays.
The classic case of an open relay is a mail server that allows sending without authentication. This is still common, as it used to be the default configuration of a server.
In the meantime, however, open proxy servers (e.g. Squid, or an incorrectly configured Apache) or so-called form mailers, i.e. scripts that send the content of forms to a recipient as email, are often misused to send spam.
You should therefore ensure that your server is not misused as an open relay. Otherwise, regular emails may be rejected by recipients or your server may be temporarily blocked.