[exim] forking a msg back to the sender

Greetings all...

Trying to build a mail router for an outgoing mail server, that sends a
copy of the message to the various servers depending on the sender
address (the configuration of the sender in our DB).


I think the system filters are the way to go, but if I use this in the
system filter:
deliver user@???


- Does that message get re-injected into exim for routing, and then
delivery?
- Or, a SMTP connection to test.com is made right there?


If the second case, it would be great to have:
deliver user@??? sender@??? 1.2.3.4

Meaning: send the message to 1.2.3.4 IP, with mail from: sender@???
and rcpt to: user@???


Anyways... Does anybody have a good hint for me here?

Thanks a lot