Intended relaying happens all the time: your mail server accepts mail from authenticated users and delivers it to other domains. A smarthost that internal servers send through is a relay as well.
An open relay forwards mail from anyone to anywhere, without authentication or IP restriction. Such servers get abused for spam within hours and end up on blocklists.
So the message Relay access denied is usually not a server fault but a sign that the client sends without authentication or on the wrong port. In Postfix, mynetworks and smtpd_relay_restrictions control who may relay.
Example
postconf mynetworks smtpd_relay_restrictions