[exim] Delivery logs when using alternative conf file

I need to use independent instances of exim on one computer, each instance
with a different configuration file specifying differents log_file_path,
pid_file_path and spool_directory.

So far i managed to achieve that using -C option and this work fine
(messages are received and delivered without problem) with the exception
that exim is not logging on the mainlog the details for the delivery
operations, only the ones concerning receiving emails.

If i start the process as:

exim4 -bdf -q30m -C /etc/exim4/exim-user1.conf

the logs related to the delivery operations are printed to the console
instead of the mainlog.

I'm aware of exim dropping privileges when the -C option is used, but both
log_dir and mainlog are writable by the exim_user directive specified in
the configuration file.

How can i capture the delivery logs??

Regards,
Ali