[exim] selecting random ip from file

Hello.

in smtp transport I'm doing something like:

interface = <; ${lookup{$sender_address_domain}
nwildlsearch{/etc/mail/custom_ip} }

which contains:

example.com: 1.1.1.1

That works fine but I would like to have few IPs assigned to a domain
and let exim choose these randomly like

example.com: 1.1.1.1, 2.2.2.2, 3.3.3.3

The number of ip addresses can be different for each domain listed there.

How to approach this in exim way?

(note file format can be easily changed like spaces instead of comas etc
if needed)

--
Arkadiusz Miśkiewicz, arekm / ( maven.pl | pld-linux.org )