I currently have a Postfix/Dovecot setup, but our server is hitting capacity, so we decided to switch to Google Apps. However, we have certain web applications that need to send and receive e-mail directly (for example, inbound e-mail being redirected to a script, and sending password-reset e-mails).
What I want to do is host certain e-mail accounts with Google Apps, while my current server continues to manage other e-mail accounts. I assume this means that I would use my current server as an Inbound/Outbound Gateway, so my questions are:
1) How do I setup Postfix as an Outbound Gateway without making it an (insecure) open relay?
2) How do I setup Postfix as an Inbound Gateway so that only certain e-mail addresses are forwarded to Google Apps?
3) I was also considering a dual-SMTP approach where both Google Apps and my server can send mail directly to the outside world, but would this be spam-filter friendly? I want to keep using a single domain for all e-mail addresses if possible.