Dec 10 01:35:29 m33 postfix/smtpd[4265]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 550 5.1.0 <slon@m33.ga>: Sender address rejected: User unknown in local recipient table; from=<slon@m33.ga> to=<slon@m33.ga> proto=ESMTP helo=<[127.0.0.1]>
'm33.ga' is your server hostname, but 'slon@m33.ga' is not a valid email account (you cannot find it in iRedAdmin). That's why you cannot send email with 'slon@m33.ga'.
'm33.ga' is value of Postfix parameter 'myhostname' and 'mydestination', so emails sent TO '[system_account]@m33.ga' will be accepted and delivered to system account.
