[Exim] Questions using transport filter

Hi

I am currently developing an S/MIME Relay using Exim as MTA.

To actually encrypt/sign outgoing mail, I use an external program which is
called by exim as transport_filter, as this seemed to me the only possible
way of calling an external program for outgoing mails (Exim 3.34).

The problem is that I seem to have no possibility of actually cancelling a
mail delivery whithin transport_filter, as the connection to the remote
mail server is already established and the Recipient already specified.

My question therefore are:

- Is there a way to cancel a delivery whitin transport_filter?
- If no, is there another way to call an external progam for all outgoing
mail deliveries?

Bye
Tim