[exim] dnsdb coversion

Hi all,

In the transport below, I am looking for the a record, so that I can set the
outgoing inerface to that record (ip).

But I have some domains that the a record is not in our servers, so I would
like to do an mx lookup instead, then, convert the mx record to an IP.

Any idea how?

remote_smtp_alias:
        driver = smtp
        helo_data = $parent_domain
        interface = ${lookup dnsdb{a=$parent_domain}{$value}{0.0.0.0}}
        return_path_add = true
        debug_print = "\n Transport:       remote_smtp_alias \n \
                       Destination:     $local_part@$domain \n \
                       Sent VIA:        ${lookup 
dnsdb{a=$parent_domain}{$value}{0.0.0.0}} \
                       Hello:           $parent_domain \n"

TIA,

-Grant