[Exim] Denying specific users use of SMTP

I have a special case which seems to be quite unique to the traditional
unix setup. My networks are used by schools. The primary server and all
workstations are windows machines, but the mail/internet server is a
Linux server. I am using winbind to sync the windows users onto the
Linux machine, to save adding new users twice. However, the problem is
that it is not always desirable to give every student in the school an
e-mail address.

We have a group that all 'Mail Users' are a part of, on the main server.
I could write a script to poll this group every so many minutes and pipe
the resulting users to a text file (or poll the non-members) but need to
know if there is a way to configure Exim to parse such a list and
exclusively allow/deny use of it's services to specific users who
actually exist on the local system?

It would need to be capable of checking a list from a separate file
ideally, since there can be over 1000 users and I don't intend to make a
full-time job out of maintaining the exim.conf file!

For info, I'm still running Exim 3.36

Thanks in advance
Richard