[Exim] Restricting From: addresses for SMTP

Hi,

I have SMTP AUTH configured (for PLAIN, CRAM-MD5, LOGIN) without a
problem, and that is working great, but I would like to restrict the
addresses that a person can send from via the SMTP server.

For example, if I have a mailbox account "user@domain", I would like to
restrict them to sending emails from "user@domain" only via SMTP.

Is there a way of altering the smtp transport or the dnslookup router to
also add in checking of the from field based upon the earlier supplied
SMTP AUTH data? The exim book mentions $authenticated_id, so could I use
something like:

dnslookup:
driver = dnslookup
domains = ! +local_domains
"from" = $authenticated_id
transport = remote_smtp
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
no_more

or can I use something with verify_sender? or something else?

Thanks for any help.

Graham