I've seen a few answers to this on the web, but nothing definitive nor any good examples.
I relay all my mail out through Postfix, currently everything sent to my ISP's mail server.
However, there are a few users with gmail accounts or gmail-hosted accounts; I'd like to route their outgoing mail through Google's servers (since it will then appear in their Gmail sent folder). All email NOT from one of these senders should go out through the default relayhost.
I believe I need to use the "sender_dependent_default_transport_maps" configuration within Postfix, but can't find a good example on what this setup looks like. Can anyone point me at a specific setup?