I need to be able to specify what interface Postfix will use to deliver outbound mail on. What I am doing is setting up an SMTP gateway to deliver mail from different, unrelated groups. I plan to setup a separate instance of Postfix for each group to receive mail, and each instance will need to deliver on its own virtual interface.
The reason I need them separate is so that if one of the groups misbehaves and gets the server blacklisted, I don't want any of the other groups affected. But I don't want to have to setup entirely separate servers or VMs if possible.