[exim] control outgoing relay

Let's say I have setup Exim to act as an outgoing relay for the domain
mydomain.tld.

The users (whitch are all system users having maildirs) authenticate
themselves via SMPT AUTH.

The problem is, that e.g. the user tom (tom@???) can use any local
part as sender address (e.g. anything@???). I want to restrict the
users to their own local part.

How can this be done?

Thanks

Raphael