Server :: Unsure Of Exim Configuration - Accepts Mail From Users On Multiple Domains To Pass On To The Outside World

I'm in the process of starting a migration from an old postfix server to nice shiney new exim server however there are a few things i'm really not clear on and i'm hoping that some one here could point me in the correct direction. Okay the postfix server at the moment does the follwoing ( i'll try and keep this simple ):

* accepts incoming mail for users on multiple domains and puts the mail in the users mailbox ( external -> internal )

*accepts mail from users on multiple domains to pass on to the outside world ( internal -> external )

Obviously the POP and Imap functions are handled by other daemons.. I'd like to replicate this sort of setup on the new exim box, however the guides i can find only help with configuring the exim system for incoming mails ( external -> internal ) for multiple domains and i cant seem to find a guide that would indicate how to do both ( internal -> external and external -> internal ) on one box. I'm guessing i may have to do some sort of auth to get exim to accept and then handle the internal -> external side of things?