So this topic does touch both programming + server world, makes more sense to post it here than serverfault.com..
My postfix server is set up to handle mail differently for each domain on the server, however each domain uses the same IP. So, when mail goes out, postfix is unable to track rev IP correctly to determine the correct domain / transport route.
So next option: some how having PHP include a header or smtp setting to say that this mail is coming from "domain.tld"; and postfix determining the route based on that.
So far, I haven't been able to figure it out yet, some fresh brains on this question would be nice.