[exim] Auth'd Mail to bucket-1, localsent to the world

Greetings,

I've dove in head first and I can't seem to get any further on this.
Something tells me it's because I don't know how to describe it using
standard terminology.

I want to accept incoming mail using AUTH and TLS - done. But I want to
process this mail first with an external program and then send it out.

How do I create the rule:
- if incoming mail is authenticated, then pipe to this script.
- if someone locally is trying to send mail using "/usr/sbin/sendmail -t",
then just send it out to the world as normal.

Help would be great. Thank you

JLC