[Exim] Help please?

No doubt this question has been asked and answered a zillion times, but my
study of the archive's hasn't produced the results I was looking for....

We acquired a domain from someone. As a result, there were a multitude of
users who are no longer on the domain but are still getting a million junk
letters a day, mostly from non-returnable email addresses. (Gawd, I hate
spammers.)

I need to set up exim to send all that junk mail to the email address
"junkmail"

so if email is going to tombrown@??? and he's not a user, instead
of bouncing it, exim will forward it automatically to junkmail@???

By going thru the archives, I noticed that changing lsearch to lsearch* and
then adding the line * : junkmail to the bottom of my aliases file.

But when i do that, ALL mail to mydomain.com is going to junkmail, not just
the unknown user email.

Any suggestions offlist would be MOST appreciated.

Thanks