This information is used to route mail to particular domains to another server.

Here is an example of routing mail to a central mail hub, like Exchange.

Note: Use a bracketed host name on the right hand side (i.e., "[foo.bar.com]" instead of "foo.bar.com") if you don't want to do an MX lookup to resolve the host.

example.com                smtp:[exchange.example.com]

# this also adds all subdomains
.example.com               smtp:[exchange.example.com]

Make sure to include a space or tab between the domain and your relay destination host.