[Exim] Aliases question

All,

The exim mail hubs are set up the local_domain kingston.ac.uk and there
is a user_aliases director which handles the forwarding of the mail to
the various mail drops. For example user@??? is changed to
user@???.

There are three routers (cut down)

intranet: sender_host_address 141.241.*.* domains *.domain.ac.uk
incoming: sender_host_address ! 141.241.*.* senders !
user@??? domains *.domain.ac.uk
outgoing: sender_host_address 141.241.*.* domains ! *.domain.ac.uk

However a message coming in for user@??? which has an alias
user@??? is then rejected as unrouteable by the three
routers as the message appears to come from an external MTA, and an
external user to an external user after the aliasing has done its work
thus being rejected by the routers described above as undesirable
relaying.

What I am looking for is a method not to a reject a message that has
been changed by a director rather than entered exim directly by a router
?

Kevin

--
Principal Systems Manager for Mainframe Systems Group part of ICT
Services
Telephone x2832 EMail K.Murray@???