Hi Johnny0000, it typically is the from address the printer is sending from. Many Xerox devices need to use valid from addresses while a lot of older devices do not.
Since you have provided no details about the setup I could also add that the from address may be valid but is not setup to relay on the SMTP server. In this case you would get that error when sending outside the domain. So if it was sending from Fake.email@Johnny0000.com it could not send an email to Some.Random@This-is-fake2.com but could send to Dave@Johnny0000.com