Suppose you have a domain name, example.org, that has a primary MX host (mail.example.org) that does most of the delivery. However, one of the users, who works at example.com, actually gets delivery of <user@example.org> at work (from the primary MX for example.com, mail.example.com). Of course, a simple .forward or /etc/aliases entry would work, but this would pointlessly push email back and forth between the two mail servers — in some cases, up to three pointless passes before the final destination! That's particularly an issue in today's SPAM-laden world. Here's how to solve this waste of bandwidth using Postfix.
This tutorial here assumes you have a some reasonable background knowledge of Postfix MTA administration. If you don't, this might go a bit fast for you.