[exim] Controlling who can use the SMTP relay

Hi, first post! ;)

SITUATION:
I have a web server with cpanel.
We have multiple client / domains on the server.
Most clients use their own ISP SMTP service.
We want to charge some clients for using our server as their SMTP relay for
outgoing email.
They would be charged on usage (monthly traffic)
The website installed on the server have scripts that send emails and this
is free for all clients. It should not be in the traffic count if possible.

NEED:
We would like to control who can use smtp.domain.com from outside the
server.
We would like to monitor the traffic and usage of the service for them.

How can we control what user use our SMTP server?

Thanks in advance
--
Thierry