[Exim] Need help (how to correctly rewrite headers)

Hello :))

Please explain me how to get correctly such effect, what to read about this,
what options in exim.conf provide this.
I have local network with Linux server and number of Windows NT clients.
Server connected to Internet via dial-up.
@mydomain.isp.net - my mail domain on ISP
@localdomain - local network domain
If mail send from one local user to another, it must be delivered directly.
If local user send mail to external (internet) address, MTA must change
@localdomain to @mydomain.isp.net in message header.
Addresses in received mail from external users must be change from
@mydomain.isp.net to @localdomain.

Thank you for help