> One of the domain owners wants to restrict certain particular users
> within his domain from sending or receiving email outside that domain.
> So, for instance, while "mark@???" might be able to receive and
> send wherever, "jeff@???" is only able to receive/send from/to
> the "domain.name" domain.
This is a job for ACLs. Put the restricted addresses into a file and this
into your RCPT ACL:
deny senders = CFG/restricted
! domains = $sender_address_domain
message = You are not allowed to send outside of your domain
deny recipients = CFG/restricted
! sender_domains = $domain
message = recipient is not allowed to receive from foreign domain
(at least something like this should world)