This is one of those scenarios with strange requirements from a client:

There are two Forests that don't want to have anything to do with each other
but they want to share the same SMTP address which belongs to the first
Forest. However the two Forests belong to the same company and they are OK
with sharing the SMTP address space.

=
FOREST-A.COM
Running AD 2003 and Exchange 2003
Exchange Org in this Forest has the Ownership of this "FOREST-A.COM" SMTP
address with about 200 mailboxes.

=
FOREST-B.COM
Running AD 2003 and Exchange 2003. This Exchange Org doesn't have the
ownership of FOREST-A.COM SMTP address space.
"We have 800 users in this Forest that need to use the SMTP address space of
'FOREST-A.COM' but use the FOREST-B.COM Exchange ORG as their mailbox
server"

There is some politics going on and these two Forests want to have no
technical TRUST or any other type of integration such as using one as the
Exchange resource forest or ...

They are currently looking into an email Gateway Appliance Box to handle
inbound mail distribution but I am not even sure if they can find solution
in this area.

*Questions - I don't blame you if you don't answer this scenario:)

How do you share one SMTP address between two Exchange Forests? keep in mind
that these two Forests want to have nothing to do with each other.
Can Front-end SMTP server help in this scenario?
Any recommendation?

Thanks in advance,
Bruno