[CentOS] How to handel smtp to public servers

Hi,

we do have some subnetworks for private computers, which are allowed to use there public smtp servers like msn, web.de or whatever with the users private accounts.

All our own computers have to send mail trough our mailserver with user authentication.

From time to time we are faced with the fact, that a virus infected private notebook sends spam and we are told by our ISP to take care :)

What might be a good choice to allow clients to send unrestricted transparent mails (= use smtp(s)) but we can monitor? E.g. like a redirect or proxy for smtp?

I like to know which private computer sends lot of mail. :)

Thanks for any hint and suggestion. /Götz