[Exim] preventing relaying from local users

I have do something a bit unusual:
I need to prevent local shell users from sending mail mail outside of
local_domains + relay_domains.

Even if I have
host_accept_relay =

Users that run "mail user@domain" invoque exim locally, and thus bypass
relay checking.

All the mail gets relayed to a relay host, so I suppose I could alter my
exim routers ocnfig to make other domains non reachable as far as exim is
concerned, but if that's my only solution, I then lose the option of having
a while list with sender_address_relay

What do you think the best approach is?

Thanks
Marc
-- 
Microsoft is to operating systems & security ....
                                      .... what McDonalds is to gourmet cooking

Home page: http://marc.merlins.org/ | Finger marc_f@??? for PGP key