Can't forward messages from internet to in-office

[CFS]

When I receive a message from the internet and try to forward it to an
in-office mailbox on exchange it comes back undeliverable. 2 computers have
this problem.
Using Outlook 2003. You can send out a new message fine but you cannot
forward. You can forward to an internet e-mail address. Exhange Server is
used as the in-office mail server. Both editions that do not work came with
the Office Student/Teacher edition. Any ideas???

Thanks