[exim] route mail based on source

Is it possible for Exim to distinguish between mail from the internet and
that from a mail client/MUA?

I want mail from the internet to be delivered locally and if it is from an
authenticated user then honor my MX record and send it to the internet.

Here's my use case.


- I'm switching to gmail and I have my MX record pointing to google's
mailservers.
- google is configured to send everything to my Exim instance.
- Exim is configured to save the messages locally

So now when someone sends mail to my users, it goes to google then to my
Exim instance. My users can view it in both places.

The problem is if my users send mail to the Exim server directly it is not
delivered to google.
Is there a way to make this happen?

Thanks in advance for any suggestions.
Tony