[exim] Routing certain users of a specific domain

Hello,

I have to configure an exotic mail server. It will be Exim3. We manage
two domains :
- example1.com : all users (*@example1.com)
- example2.com : certains users (john@??? and
foo@???)

                                +-----------------+
MAIL TO: john@??? ---> | our mail server |---+
                                +-----------------+   |
                                         ^            |
                                         |            |
                                         +------------+
                                          redirect to
                                          john@??? because it
                                          has john@??? as alias

All mails to example1.com "belongs" to our mail server. For
example2.com, mails must been relayed to MX of example2.com except for
certain known users.In local, john@??? and foo@???
will be alias to the example1.com domain.

How to specify this type of configuration to get this behavior ?

Thanks
JR.