[exim] Total novice - autoreplies for aliases

Hello!

I am totally new to exim and am admining an office system which is using
it and have been asked to set up an autoreply for a defined set of
aliases. I taken a look through where the router and transport configs
are kept, but am not up on the language yet.

What do I need to do in order to make an autoreply for a list of
addresses which are aliases. We are using ldap front end to maintain our
user list. There is an /etc/aliases file in existence but not all
aliases appear in there.
We also have a vacation autoreply set up, but this does not appear to
work as a solution as I have tested to the aliased addresses and have
received nothing, and from the exim log I can see the message going to
the recipient still.

Where do I start?