[Exim] Creating Mailing Groups

Hi there,

I have a Linux server running Debian 2.2 and Exim 3.12-10.
My clients run Outlook 98 on a citrix server and send
and recieve their mail through the linux server. I would like
to know if it is possible to create mail groups. one for users
allowed to relay through the server and one allowed to send
local mail only.

We have 180 users using this server and we would like to
have all users sending mail locally but only selected users
sending mail externally.

Is there any documentation on this feature or is there an MTA that handles
this type of feature specifically?
Any help would be appreciated.

Many thanks
Gary