relay as any email they want ?

[dgsdgs]

I got a problem with some users who uses my ispconfig server as a relay for domains that are not hosted on the server ex. user1@domain1.com, can send/relay mails from user2@domain2.com, I think this is a big problem, because it means that the user can relay as any address they want, as long as they are authenticated, I would like that users only are allowed to send email from the emailaddress they use to authenticate against the smtp server. Is this possible ?

I have found some thing about a postfix option called "reject_authenticated_sender_login_mismatch" and "smtpd_sender_login_maps", but i can't get it working.

I hope someone in here can help.