I'm running the Postfix/MySQL according to the ISPConfig3 setup with multiple virtual domains. I'm now using an email filtering appliance to filter email before delivery to the mailserver for some domains. So, for example, I have sample1.com's mx record pointing straight to the mailserver, while sample2.com's mx record points to the email filter appliance, and the appliance has a transport rule to send sample2.com's email to the mailserver after scanning. Now I want to restrict postfix on the mailserver to only accept email from the email filtering appliance for sample2.com, so that spammers cannot circumvent the filter and send to users on the mailserver for that domain. Would someone please provide a specific example for Postfix's configuration to restrict a specific domain to receive email from only one IP address?