[exim] Filtering Within Hosted Domains

Hello All.
I hope someone has an answer to my problem. I'm running MailScanner, Exim,
MySQL, Courier-IMAP. The MailScanner is located at a different server
while both EXIM/IMAP on another. One of our virtual email customers
doesn't want to receive emails from an address on another domain that we
also host on the same server and instead wants them forwarded to one email
account.

Condition: From user1@??? and To: *@domain2.com
Action: Forward to user3@???

I already got an answer from the good people at the MailScanner mailing
list but unfortunately it doesn't apply to our setup. I already have a
rule like this on our MailScanner server. But the problem is since all 3
domains are located on the same server, emails don't get sent to
MailScanner anymore since all domains are considered local, and thus rules
cannot be applied. Is there a way I can do this using Exim?

Thanks in advance for any help.

Cheers,
DOODS