You should be able to send emails to valid users, that's the normal operation, even if you're sending it as a non-existing user. Postfix will catch the mail and relay it if the user exist on your MTA or in your relay_recipient file.
It will even allow emails sent from a valid user to another valid user. If this email is indeed a spam, MailScanner will pick up on it and tag it appropriately.
It will not allow you to send a test email from any domain to another domain outside of the ones you're relaying for. You should get relay access denied when trying this.
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