[exim] [Newbie] Using To: header to replace envelope recipie…

Hi,

A newbie question.

I have two domains, say foo.com and bar.com.

For business reasons, I can't change the MX records
for foo.com.

All mail for addresses at foo.com is sent to foomail@??? by
the DNS provider.

The To: headers are preserved in the messages.

I would like these foo.com messages to be delivered to the
users at bar.com.

I can modify the exim configuration at bar.com.

Is it possible to extract the local part from the To: header
and use it for delivery?

i.e.

fred@??? would arrive as foomail@???
and be delivered to fred@???.

bob@??? would arrive as foomail@???
and be delivered to bob@???.

Thanks in advance for any help you can offer.

Best Wishes,
James