[Mimedefang] PGP automation?

[Gary Funck]

Consider a situation where various recipients of
mail need to send and receive messages in PGP-encrypted form
between each other.  They all have individual PGP keys.  There is no
need to encrypt messages between recipients who reside ont the host system
that MdF manages.  I was wondering if there is an effective,
easy to use method of implementing the PGP-based encryption
and decryption at the mail demarcation point, without users of
the system having to take any explicit action?  One implementation
issue is how MdF, residing on the mail server, might have
access to the private keys needed to encrypt and decrypt messages.
Knowledge of pass phrases is even more problematic.  Note as an
alternative, for the particular application being considered,
it would be okay to have a project-wide key pair and thus
handing the private key over to MdF would pose an acceptable risk.

Any other ideas for implementing PGP automation that is mail
client agnostic and requires little/no user intervention would
be appreciated.