You can test via telnet.
SSH to your box and send test email to a real icloud recipient.
Read here: How to Test an SMTP Server with Telnet. Step-by-Step Tutorial
telnet mx01.mail.icloud.com
Or tail the mail log when sending from MIAB and paste here the response:
cat /var/log/mail.log | grep "postfix/smtp" | grep -P 'status='