How can I send emails from multiple relay

Hello community. I have multiple virtual servers inside a VPS that’s running latest Ubuntu, Postfix, and Virtualmin/Webmin.

I use mailgun to relay my outbound emails from the server, but now I have another virtual server that needs to use its own and private relay through a different service.

How can I set up site1.com to use my personal smtp_relay, but let site2.com to use a different smtp relay set up?

Thanks guys.

See