Restricting what users can send mail to off-site destinations

Question

How can i restrict what users can and can not send mail outside mi local domain ?? Without editing postfix ??
Tx

Answer 1

Without editing postfix? pretty much impossible.
Anything you do is going to change posftix's confguration, and you're asking for options beyond Apple's Server Admin GUI, therefore you going to have to do some under-the-hood tweaking of postfix.
(just to be pedantic, using Server Admin ultimately edits postfix's configuration files, so what's your reluctance here?)
As for your request, there are so many ways of implementing this. The 'right' option depends on more details than you've specified. For example, how are you determining which users can/cannot send outside mail? Is it a fairly static list, or does it change frequently?
The only option that comes close to not editing postfix is authentication. By default, only authenticated users can send outgoing mail, but anyone can send mail to a local address on the server, so that gives you a simple approach - have the permitted users configure their mail client with a username and password for SMTP, and have the non-permitted users leave the SMTP username/password empty.
That's hardly foolproof, though - if any 'non-permitted' user enters their username and password they automatically fall into the 'permitted' group. That may or may not be sufficient for your need.
There are other options, though, including IP address range checking, a permitted senders list, port options, and more. It's just a matter of how much you want to tweak postfix's configuration.