+A complex example, using a file like:
+.code
+alice@dom1
+bill@dom1
+maggie@dom1
+.endd
+and checking both domain and local_part
+.code
+domains = ${domain:${lookup {$local_part@$domain} lseach,ret=key {/path/to/accountsfile}}}
+local_parts = ${local_part:${lookup {$local_part@$domain} lseach,ret=key {/path/to/accountsfile}}}
+.endd
+