[exim] Pipe message and auth login to a external script

Hello, I've been doing a bit of research but I'm stuck on this one.
I'd like to be able to get a message sent by an authenticated user and
pipe it to a script, along with the auth login of the sender. I'm
using plain authentication, and I've tried using $auth1-3 variables
but they don't work, if I try to print them they're empty. I'd like to
be able to do something like this:

[...]
begin transports

pipe_delivery:
driver = pipe
command = /bin/bash -c "/usr/local/bin/myscript $auth2"

Is it possible to do this with Exim? Any workarounds?

Regards.


--
Luis