I have an Ubuntu server setup as an email server running postfix. This server sits on a public IP address and handles mail for several domains. I also have a Windows server inside my firewall running Mail Enable as a list server for some of our alarm systems. Our internal alarm systems email an address on the Mail Enable server and that email gets sent to all the facilities personnel.
I need to be able to forward/relay mail from my public mail server to my internal mail server so it can go to a distribution list.
What I would like to happen is: all mail go to my public mail server (lets call it mail.foobar.com) and if the user exist on that server it is delivered to their mailbox. If the user is not located on that server I would like for the mail to be forwarded to my internal list server. So when mail comes to email@example.com and Bills account is located on the server mail.foobar.com it is delivered to his mailbox. But if an inbound email comes into mail.foobar.com for the user firstname.lastname@example.org and her account is not located on that server then it gets forwarded or relayed to mail2.foobar.com.
I know I'll need to open a port on my firewall to the internal server and I'll need to setup and A record for that server but after that I'm lost.
Can someone point me in the right direction??