Prevent users sending mail to certain external domains.

[airtonix]

We run zentyal server in a small business environment, where it provides most of the services zentyal can provide.

We also have several less than sensibly attentive employees that do not pay attention to whom they are sending emails (half of the time).

What I would like to do, is configure a mail filter rule in zentyal that prevents anyone from sending email to a certain domain (of which i have no control of ).

[christian]

White-list or blacklist for outgoing mails...
For what I understand, this is not so easy: you will either have to define the list of authorized domains or defined list of forbidden domains, and also decide whenever this rule applies to all users or only some users, which would mean to force authentication before sending.
In any case, I don't think such feature exists in Zentyal interface. I'm afraid that "external connections" in "mail filter" interface is only for incoming flow.
You will have to tweak Postfix config and use, e.g. "smtpd_restriction_classes"

Have a look at: http://www.postfix.org/RESTRICTION_CLASS_README.html
and also this: http://www.freespamfilter.org/forum/viewtopic.php?t=659