To resolve the issue, allow connection from internal servers:

1. Make sure that the IP of the MailServer is added in the approved list for IP Filtering. From the console, go to IP Filtering > Approved List to check this.
2. Back up /opt/trend/imss/postfix/etc/postfix/main.cf.
3. Open /opt/trend/imss/postfix/etc/postfix/main.cf and look for the "smtpd_client_restrictions" key.
4. Add the "permit_mynetworks" on the "smtpd_client_restrictions" key similar to:
   smtpd_client_restrictions = permit_mynetworks,
   hash:/opt/trend/imss/postfix/etc/postfix/NRSAllowAccessList {...Other original values}
   (permit_mynetworks has to be the first value in the list)
5. Restart Postfix. Check this Postfix readme file for more details about Postfix. Follow the instruction for mynetworks_style (default: subnet).