[exim] Rewrite FROM address

Hello,

Another quick question, I personally utilise receipent forwarding or
whatever it is called, i.e. Luke+forumname@??? will come to
luke@???

However to reply to groups, I have subscribed to using this method, means
the mail has to come from that address. The way I am doing it, is to setup a
new imap account in my email software. However this is getting all a bit
much :P

Is there a way to get exim to rewrite the from, if the to is a paticular
address. I.e. If a mail comes from luke@???, destined for
user-forum@???, it can rewrire it to luke+somesite@????

I am guessing it can, but havent a clue on the config.

Cheers