[exim] loop detection

Hi,

I have few routers set up in such way:

- alias (routes user@domain to system_user)
- forward (routers system_users to additional something@???,
other@???)

Now if user sets up a loop like this:
user@domain -> system_user -> forward to user@domain
then exim will report this as ,,Unroutable address''.

Is there any nice way to detect loop at last router (or any router) (and 
return other error message than ,,Unroutable address'')?
-- 
Arkadiusz Miśkiewicz                    PLD/Linux Team
http://www.t17.ds.pwr.wroc.pl/~misiek/  http://ftp.pld-linux.org/