Server :: Sendmail To Verify Sender Enveloppe Even Though SMTP AUTH?

I got one last stop until I've fully configured my Sendmail server. I want the server to verify which address the sender is using when sending mail. Even though authenticated via SMTP AUTH or mailing from an IP listed as RELAY in access.db, the user should not be able to use whichever address he likes.

Ideally, I would like Sendmail to use a file that looks something like this: Username sername@Server.org

where, if logged in with Username, the only accepted sender address is Username@Server.org. As of now, Sendmail just verifies that the sender domain exists and then relays which I don't like. This also applies to a lot of other servers as well... for example in the cases where ISPs have blocked outgoing port 25 and let all their users use their SMTP server. I assume, if they don't use SMTP AUTH which is not the case where I am, that they have their entire IP range listed as RELAY and therefore, using my ISPs SMTP when in their net, I can send mail as whomever I like as long as the sender domain exists.

I've had a look at filters but haven't found any suitable and I assume that this might be realised through a Ruleset or something. What do you think? Have I missed something making this a really stupid suggestion or am I on to something? It would make my day