[Exim] Billing and Exim

Hi,

I'm trying to figure out a solution to be able to bill people sending
emails through an MTA.

The idea is that the MTA could get the ip address of the sender to
confirm their from address address (I know this for sure) and then
before sending the message out, would pass a copy of the message to a
script which would analyze it's to, from and size before writing a
billing record.

Exim looks like a possible candidate to allow me to create such a
thing. From what I've read so far it looks like I could create a router
which would pass the message to a script which would do the address
check and possible re-write then the billing piece and add a flag
before inserting the message back to the top of the queue, or passing
it to the next router.

Does what I say sound feasible or am I barking up the wrong tree?

Zac Tolley