module can not be used at a relay

to use this module on a gateway/relay it must also managed `InternalHosts`.

Default content (worked also without wildcard support) can be

```
127.0.0.1
localhost
$::network_eth0/$::netmask_eth0
```

Not sure if it better to use `refile:/etc/opendkim_trustedHosts` to allow wildcards.

To enable this kind of function a flag should be included (like `relaymode`) which has `false` as default.
