Sender Dependent Outgoing IP Address
https://www.virtualmin.com/documentation/email/dependent
This behavior can be changed on systems running Postfix version 2.7 and Virtualmin 3.94 or later, so that outgoing email from a domain with a private IP address appears to come from that address. This can be useful for separating email from multiple domains as seen by other mail servers, or for setting up per-domain reverse DNS records.
requires Postfix 2.7 (seen on most modern Linux distributions), Virtualmin 3.93 and ideally Webmin 1.600. The steps to set it up are as follows..
Have not actually done this.