I am familiar with customizing the sendmail delivery part, with
procmail or your own delivery agent.

The part I am curious about is the other side. Is there anyway (other
than rewriting sendmail) to place a preprocessing filter ahead of
sendmail? The filter in question will look at the From and To headers
and use pgp to selectively encrypt some of the mail messages.

Is there something already out there which does this?

-Ramon F Herrera