Your mentioned email is rejected by IronPort since it is considered an incoming email and the recipient address is not listed in 'Mail Policies'-'Recipient Access Table' (RAT).
Since the sending host is not listed in RELAYLIST or any sender group using RELAYED mail flow policy or Relay connection behavior (and the SMTP connection is not SMTP authenticated), it is considered an incoming email connection. If the recipient address is not listed in RAT, it will be rejected (avoid open relay).
