[exim] Different local domain for bounces

Good morning,

I am running Exim 4.80 on Debian Wheezy.

I have setup a "maildrop" with Dovecot, Exim and Fetchmail, where an
internal server fetches mail from external POP3 and IMAP servers, stores it
locally and serves it over IMAP to local Samba4 AD users. So far it works
great. Now, I am trying to setup an SMTP relay so those users can point
their mail clients to this internal server in order to *send *mail. I have
configured Exim with my ISP's SMTP server as a smarthost, and it works fine.

The missing part is correctly handling the bounces in case the ISP's SMTP
server is down, for example. *I need all mail coming in from the users' PCs
via port 25 to be relayed to the external SMTP, regardless of their
destination (even for addresses belonging to the local domain) *(since some
mobile users access the external mailbox directly).

Let's say my domain is foo.com. If I set foo.com as a local domain, then
mail from bar1@??? to bar2@??? gets delivered locally and never
reaches the external SMTP server (not what I need)

If I don't set foo.com as a local domain, mail reaches the external SMTP
and gets back properly, but if the external SMTP is down then the bounce
goes nowhere (it is obvious since it doesn't know it is a local mailbox)

How can I get those bounces to get locally delivered? I guess I need some
way of specifying that foo.com needs to be considered as a local domain
just for bounces, but regular mail to that domain needs to go through the
external SMTP server.

Thanks a lot

George