[exim] Multiple servers and IPs for outgoing mail

Hi,
I am looking at implementing the following setup, but was wondering if
anyone had any advice for this.

I have 100 high volume senders, and would like to split them on to 3
different servers, with 1/3 on each server.
Each of the senders needs the ability to send from their own unique IP
address.

Now, I'd ideally like all of the senders to be able to use the same SMTP
server name of (for example) smtp123.com

I am thinking that putting a roundrobin DNS on smtp123.com could be part of
the solution, so that each server automatically receives 1/3 of the attempts
to send.
However, each server would then need to route the outgoing email to the
actual server that should be used to send the email (i.e. the server that is
setup with the IP address for that particular sender).

Does anyone have any experience with this?
Thanks very much