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?