Add configuration parameter defaulting to 'NO'

Adds optional parameter to limit the alias destinations to local domains.
An additional check makes sure that the domain is in the list of postfix domain names.

config.inc.php

@@ -505,6 +505,10 @@ EOM;
 // address is legal by performing a name server look-up.
 $CONF['emailcheck_resolve_domain']='YES';
 
+// When creating mailboxes or aliases, check that the domain-part of the
+// address is local and managed by postfixadmin, preventing remote domains
+// from being the destination for an alias
+$CONF['emailcheck_localonly_domain']='NO';
 
 // Optional:
 // Analyze alias gotos and display a colored block in the first column