Question about using external email gateway (mailcleaner)

I have installed mailcleaner for extra email protection. Everything almost finished except one point.

  • Callout Connector to verify email address inside Virtualmin by using SMTP

Setting on mail cleaner:
Callout connector: smtp
Use alternative server: (private IP address)

Test configuration:
Sending to random recipient: Recipient correctly refused
Sending to postmaster: Could not setup recipient <— Does anyone has experience on it?