[exim] Rewriting Reply-To header to match From Address??

Looking into rewritting the Reply-To header...

Some info first:

We have a gateway server which does all the Internet mail transfers both in
and out. It has a number of domains it provides mail services for.

On some of the domains, the email is forwarded to specific user accounts to
an Inside mail server using the that mail servers hostname and the specific
userid for that mailbox on that server (not always the same as the inbound
local part for that user using their domain).

So, lets say we have a user name fred with an Internet Email address of:
fred@???

Email arrives and gets queued for the inside mail server for
fred123@???

Everything is cool... The user gets the email using whatever protocol they
use and when they send email, it authenticates with the inside mail server
with that same address, and sets in the email their Internet email address (
fred@??? in this case).

When it gets to the outside mail server it looks like:

Reply-To: fred123@???
From: fred@???

However, once the message leaves the outside mail server, the Reply-To
header is bogus since there is no Internet known host called
inside.zipturtle.com so it would never get back to the gateway server
should it have to go several hops.

All of the examples that I see of rewriting deal with the same domain. In
this case the inside and gateway server using a completely different domain
than the email address involved. The From address is correct, but the
Reply-To headers make no sense after the email leaves the mail gateway.

Hope this makes sense.