Many people want to get a free ride from your system by using it for relaying their mail. This can be due to them being corrupt and wishing to let you take the rap for relaying their junk, or them being lazy and unable to make their own systems work. In any case this is a theft of service and needs to be stopped.
Exim v4.x has completely different and considerably more flexible mail policy controls than earlier versions. The documentation is also considerable more complete and the default configuration file has a well commented section on relay control (in the RCPT TO ACL. You should instead read the specification section on Access Control Lists.
These are fully detailed in the Exim Specification Document. The specific section on relaying is here
The standard settings for a workstation, allowing relaying through the loopback (since packages such as MH post mail this way), would be:-
relay_domains = no_relay_domains_include_local_mx no_relay_match_host_or_sender host_accept_relay = 127.0.0.1/8
this is actually the default settings other than that for host_accept_relay.
The information to do more complicated manipulations can be found in the specification document and is outside the scope of this note.