Ramon F Herrera wrote:
> 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.

Hunh?

Given that pgp/gpg is rather easy to use from a MUA, I'm not sure
why you'd want to do this..

>
> Is there something already out there which does this?

Yes. Sendmail, mimedefang, spamassassin.