[exim] restrict smtp auth to certain users/domains

How can I allow only certain e-mail users (or domains) to use SMTP
Authentication?

This is what the relevant section in my exim.conf currently looks like:

  accept  hosts = +auth_relay_hosts
         endpass
          message = $sender_fullhost is currently not permitted to \
                        relay through this server. Perhaps you \
                        have not logged into the pop/imap server in the \
                        last 30 minutes or do not have SMTP Authentication
turned on in your email client.
          authenticated = *

  deny    message = $sender_fullhost is currently not permitted to \
                        relay through this server. Perhaps you \
                        have not logged into the pop/imap server in the \
                        last 30 minutes or do not have SMTP Authentication
turned on in your email client.