IntroductionSuppose you want all your web servers to locally send all email (maybe from your contact forms, or whatever) to a real smtp gateway. If you're running postfix as your mta, this is quite easily to achieve.
ConfigurationIn your main.cf file (usually /etc/postfix/main.cf or /usr/local/etc/postfix/main.cf), specify your transport maps:
transport file (usually /etc/postfix/transport or /usr/local/etc/postfix/transport), specify your gateways per domain. This allows you to specify with regular expressions which emails go where. In this case, an asterisk specifies every domain (the transport file allows very complex setups, this is of course an extreme and trivial example):