[exim] Unique Relaying Problem

I have recently consolidated two email servers into one bigger email server.
The new server has its own new IP, to make things simpler for a few thousand
users, I used Xinet to forward requests coming to the old servers to the new
one. It all works great... except:

I trust the servers on my local IPs, and when an outside user connects to
one of the old boxes and is forwarded, the new box sees the connection as a
local connection instead of an outside user - that bypasses the normal relay
filtering and I am now a semi-open relay.

Does anyone have experience with how to plug that hole? I can remove the
trust for those machines, but then local users can't send email to far away
places if they still use the older ip addresses.

Thanks for any thoughts you have.

Bill