[exim] Alterating / Intercepting bounce messages

I was just wondering if anyone has dome this and if so - how.

I run a front end spam server for other hosts that I have no access to.
The mail comes in to my server (mx) and I spam and virus filter it and
hand it iff to the destination server. If the message is rejected there
- I don't know about it.

I'm thinking that I'd like to have bounced message come back to my
server by altering the return path. At that point I would use it for
something - not sure what yet - maybe just save them to see what isn't
being delivered - or whatever - then I want to restore the original
return path and send the bounce back to the original sender.

So - has anyone done this? And if so - can I have the code and tell me
what you use this for?

Thanks in advance.