[exim] A few questions about BATV

Thanks for the answers to my previous question regarding backscatter
spam.

I have read the links supplied in the answers and am still considering
my options.

My setup: Email-gateway on a university campus where my three servers
deliver between 160000 and 320000 emails per day. Internally most of those
emails are delivered to Exchange servers.

We also have a number of relay domains (schools and institutes) for
which we handle email. The users of those domains are not known to
us. Some of the administrators of the relay-domains indicated that
they want spam to be stopped at smtp-time. Those domains are listed
in a flat-file.

I am using postgresql as database for recipient verification and
delivery destination. Each user has indicated whether he/she wants
spam to be deleted or just marked and sent through.

Now my thoughts about the possible usage of exim's BATV-functions in
this setup:

1. Add a field to the emailuser-table in postgres containing a private
signature for each user and making sure that for all new users
added to the system a signature is added.

   Questions: 
   * What type of tool would you use to generate those keys? (md5sum?
     gpg?  sha1sum?)
   * Would it be possible to configure exim to generate a key for a
     user on the fly and store it in the database if the user does not
     have already one?

2. How should one handle email from mailing lists at the Sympa server
on our campus?

3. How would the email from the relaydomains be affected? Can the
same service be provided to them by building a database of users
who sent out email from them and add private keys on the fly to
those users?

4. Should we also, as is the case with the users' spam-handling
option, give the users a choice whether they want to implement this
mechanism or only activate it where users complain or implement it
for everyone?

I will learn from your experiences regarding these issues.

Regards
Johann
-- 
Johann Spies          Telefoon: 021-808 4036
Informasietegnologie, Universiteit van Stellenbosch

     "For what shall it profit a man, if he shall gain the 
      whole world, and lose his own soul?"      Mark 8:36