[Exim] smtp auth

Hi

is there a way to allow a user to send mail through our server without
putting his ip on sender_net_accept_relay?
we have many users from a lot of networks and it's not a good solution to
put every network on this exception list. I think smtp auth would be the
best solution, so the users can authenticate theirselves by giving their
username and password. I read the exim documentation but I didn't find
anything. Is there a way to do that? or is there a better solution to
filter, who we are relaying for and who we are not?
it's important that our users with mozilla and outlook also can use the
service.


thanks in advance


Gabor