[Exim] Storing mail in my own way

Hello.

I want to combine exim with my own mailing mechanism (based on AOLserver
and SQL+files storing).

For now I'm using smarthost, my own director and send the data through
TCP to the daemon listening on 127.0.0.1:20111.

This works fine, however I wonder if I could make exim send error
messages, that is 'no such user', 'quota exceeded' and so on...

I read something about pipe command returning non-zero error codes
(which I can get via the TCP connection), but I don't know where to
begin reading.

--
WK