How restrict senders on postfix?

[thavaht]

I know there’s a lot of material on this matter over there, but every time I follow them I get lost and don’t find clear explanation.
I recently moved from one provider to another. The DNS’s are maintained by the provider and I found that there are some miss configurations and it is taking hard to fix.

Meanwhile, because I fear being blacklisted, I want to prevent my mail users, but postmaster, to send mails to off-site destinations and I’d like to send the users an excuse message too.
Can some one provide a clear example on how to achieve that?
I’ll appreciate any help.

[falko]

I think you can do something like this with the smtpd_client_restrictions parameter: http://www.postfix.org/postconf.5.html