It means that the mail server is configured to deny relaying. Admins do this nowadays because spam and virus messages are such a big problem, not to mention the problems which may arise if someone sends a message using your server, claiming to be the ceo or some other officer or employee with some questionable content.

What it means is that the admin of that server does not want you to use it for the purpose you are trying to use it for, and they have configured it appropriately, or that is simply the default, which is the case on a bunch of mail servers today, like postfix for example.