[Exim] An intelligent 'mail-drop' account in exim 3.x or 4.x

Hi

Our school server has exim 3.36 installed (perhaps to move to 4.x) on a
FreeBSD 4.5 box, but has yet to be put in to service.

Before we go ahead, we are looking at some restrictive policies for our 200
or so pupils, most of whom should not have full email access, but
increasingly need a means of transferring homework documents to and from
school without resorting to floppy-disks.

I would like to set-up a "homework" account on our registered domain that
could be used in conjunction with a simple database that stores local
addresses against a personal home email address for each pupil. The way I
would like this to work is as follows:

local_user_A sends email to the homework account with their homework
attached, while at school.
exim picks this up and redirects it to the personal email account
remote_user_A stored against local_user_A in the database, changing the
sender address to the homework address in the process.
pupil picks it up at home as usual via their personal email account.
pupil then sends a new email or replies to the homework address with their
new homework attached.
exim picks up the email, redirects it to local_user_A, changing the sender
address to homework once more.

The idea is that no other Internet mail should arrive at local_user_A's
account and that they are unable to send to any other Internet account other
than "homework." All such mail should be refused by exim at some stage in
its processing of the message.

Staff, on the other hand would have full Internet email access via the same
server.

Any ideas?

Best regards and thanks for any ideas you could offer.
Martyn Hill
Network Administrator
St James Independent School
London