What is "my_custom_transport.example.com" ? A subdomain?
Yes. In addition, this list uses example.com as a basis quite frequently.
You didn't provide your own domain, so J.P. used this as an example.
As a side note, it doesn't have to be a real sub-domain. When the mail
is handed off to postfix my_custom_transport.example.com will be found
in the transport_maps table since you defined it there (or will soon
enough) along with my_custom_transport. From there postfix will try to
deliver the mail using the definition of my_custom_transport in
master.cf. I'm sure some folks can get into more details than I can on
what really happens, but in a nutshell this is how things will flow.
When you create your pseudo domain for use in this setup, it's probably
best to use a non existent sub-domain or some variant of your real
domain just to help avoid confusion or complications down the road. This
is not guaranteed to prevent problems, but in my opinion it's a good
first step to avoiding them. I haven't personally seen a best practices
guide for these sorts of situations. Just don't call your pseudo domain
(gmail|yahoo|hotmail).com or some other real domain that accepts mail.
