I've been moving my domains over from a server that uses Exim. With Exim, if you have a domain for which the server is the Web server but NOT the email server you put the name of the domain in /etc/remotedomains and take it out of etc/localdomains. In this way the EXIM server knows not to deliver email from someone on the same server but rather to send it out to the Internet.
What would be the method of doing this in Postfix?
One of my domains uses Google apps for email but sends out email from Dadamail based on the server. Currently this means confirmation emails are getting stuck in an account on the server rather than being sent out to the Web (and ultimately finding Google)