[exim] restrict email destination for unauthenticated users

We're looking at changing mail servers from MailEnable to exim and the
way our SMTP server is setup at the moment, any unauthenticated user
sending through our mail server can only send to to other mailboxes on
the same server. They can only send to mailboxes on other domains if
they use SMTP authentication.

Is it possible to do this in exim? and can it be done on a per domain basis?

TIA