Test how exim will route a given address.
exim -bt alias@localdomain.com
returns:
user@thishost.com
<-- alias@localdomain.com
router = localuser, transport = local_delivery
exim -bt user@thishost.com
returns:
user@thishost.com router = localuser, transport = local_delivery
exim -bt user@remotehost.com
returns:
router = lookuphost, transport = remote_smtp host mail.remotehost.com [1.2.3.4] MX=0